Drag on the canvas to draw a box.
This is a physics playground. You draw shapes on the canvas and a rigid-body engine works out what happens: falling, colliding, stacking, rolling and toppling over. No server does any of the calculating — everything happens in your own browser, and nothing is stored that you do not save yourself.
Pick a tool on the left and drag on the canvas. With the hinge and the spring you build moving constructions: a wheel, a seesaw, a pendulum or a cart with a motor. The knife cuts an object in two. On the right you set what things are made of — density, friction and bounciness — and how strong gravity is.
With the laser tool (L) you place a beam source: drag the direction it shines in. The beam reflects off a mirror and refracts through glass, exactly as in real life — how strongly it refracts is set per object with the refractive index. Switch on “Splits colours” for glass and a white beam fans out into colours in a prism. Under the Light heading you choose what each object does with light.
With the water tool (W) you drag a tank or a puddle full of liquid. The water flows, sloshes and spills over an edge, and it works both ways: a light block floats, a heavy block sinks, and a block dropped into it pushes the water aside. How heavy the water itself is you set on the right — wood floats in water of 1.0, steel sinks. The eraser removes water just as well as objects.
There is more behind the last three toolbar buttons: the gear, the chain, the rope and the glue are grouped there, next to them the rocket engine, the gravity well and the wind zone, and in the third group the tape measure, the graph and the trail. You drag a rocket engine onto an object in the direction it pushes; it fires for as long as you hold its key down. Push exactly through the centre of mass and the thing runs straight; a few centimetres off and it spins — that is not a fault but a lever.
The gear tool couples two hinges into a transmission: the ratio follows from the wheel radii so the teeth run in step, and in the panel you can flip it into a belt drive. The trail draws the path of a point, so you can see a throw or a pendulum laid out. The centre of gravity turns an object into a star the rest orbits; switch gravity off when you use it, or everything just falls.
There is measuring to be done too. With the tape measure (M) you drag from point to point and read off distance and angle; it snaps to objects, to the grid and to other tapes, and two tapes sharing an end point also show the angle between them. With the graph (D) you tap an object and see its position, speed or energy plotted against time. Press P to turn on arrows: first speed only, then the forces as well — gravity, normal force, friction and the resultant, each in its own colour and at the point where it acts. Under Measure in the panel you find the numbers for the selected object.
And there is plenty to BUILD. Every object sits in one or more collision layers (A to E): two objects only collide when they share at least one layer, and that is how you let a wheel turn inside a frame. On a hinge you set one key for anticlockwise and one for clockwise — a body, two wheels and two hinges together make a car that drives with the arrow keys, and the "Brake when no key is pressed" tick box decides whether it brakes or rolls out. On a phone the buttons appear by themselves at the bottom of the canvas. Every connection can be made breakable: you set the force above which it snaps, and the panel shows what it is taking right now. And with the wind zone (N) you drag a rectangle in which the wind blows; under Scene you find the wind for the whole world, which carries the water surface along as well.
And you can EDIT what is there. The fourth fly-out button holds move (H), rotate (O) and scale (I). Moving drags an object along with all of its connections — including an object pinned down with a nail or a hinge, because those points simply come along; hold shift to move just that one object. Rotating turns about the axle if the object has one and otherwise about its own centre, in fifteen-degree steps with shift. Scaling is done at one of the four corners. Nothing can be dragged through anything else: if it runs into something, it stops there. Ctrl+C, Ctrl+V, Ctrl+X and Ctrl+D copy, paste, cut and duplicate a whole construction, and the panel has sliders for the angle, the initial speed and the spin of the selected object.
Two buttons change the substance itself: "Turn to liquid" turns an object into water with the same area, and "Freeze water into ice" turns a pool back into an object. And with the rewind button at the top (or the comma) you wind time back a stretch — up to ten seconds, as long as you have not edited anything. That is something other than Ctrl+Z: rewinding undoes the physics, undo undoes your edits. With "Mirror" you flip a whole construction: the left half of a bridge becomes the right half.
And you can turn it into a GAME, using tools that are useful for anything. With the gear (J) you draw a real toothed gear; it gets an axle straight away, and the coupling (X) makes two of them turn together. With the counting gate (Y) you drag an area that counts whatever passes through — that is your scoreboard. Switch on "Destroyer" for an object and whatever it touches is gone: walls you must not hit, or a line that clears away a failed attempt. Give every object a label and you have names, targets and points on the workspace. Together with the steering keys on a hinge, that is all you need to build your own course or goal game.
Behind the fifth fold-out button is everything you need for a real game. With the steering head (Z) you turn an object into something that drives by itself and turns left and right on two keys; while placing it, drag the direction it should set off in. Four kinds: driving (Kurve), pushing (Pong and football), sliding and right angles (Snake). Switch on "Deadly" for that object's trail and the line it leaves behind becomes a line nobody may cross — with gaps in it, because without gaps the field is closed after twenty seconds. Switch on "Wall" and things really do bounce off it. The scoreboard keeps track of who is taking part, who gets points for what, when a round is over and who wins. You choose keys by pressing them; on a phone two coloured buttons appear at the bottom for each player. The button at the top switches between PLAY and BUILD: in play mode the letters steer the players instead of the tools, and Escape puts you back in building mode.
Under Scene there are three settings that make building itself easier. The snap grid makes every shape you draw land on whole centimetres or metres, so ten identical bricks really do line up. The world edges decide whether there is a floor with side walls, a floor only, or nothing at all — you need that last one for orbits around a gravity point. And the tempo now goes down to ten times slower, so you can follow a collision or a collapsing bridge step by step. Every number beside a slider can be clicked and typed in when you want an exact value.
Behind the menu button at the top right are fifty-one examples in ten FOLDERS: to get started, measuring and graphs, water, light, drivable vehicles, structures, force and wind, editing and time, games, and challenges. Click a folder to see what is in it; the button at the top takes you back to all folders. Every example shows one thing and starts running straight away - only the scenes with keys wait for you. At the top, each example shows an explanation of what there is to do: which key, what you can cut, what you can drag. It stays until you do something yourself. In the Challenges folder every scene has a goal: drive to the finish, get the marble into the bin, knock the tower down, aim the beam at the target, lift the boat through the lock, put the bucket on the pedestal. An example is just a scene: you may change anything about it, and Ctrl+S saves your own version.
Holding the right mouse button and moving grabs an object physically, whatever tool you have selected. On a phone a long press does the same. The wheel or two fingers zoom; the middle button or two fingers pan the view.
You drag a rectangle onto the canvas and release; the block falls, tips over the edge of the one below it and comes to rest at an angle. That is the whole interface. There is no start screen and no empty world: gravity is already on, something is already there to knock over, and you can get at it straight away. You draw a ball the same way, with the second tool. If you want to move something while everything keeps running, you grab it with the mouse — the tower then leans under your hand, exactly as it would in real life. Pause, one step forward and back to the start sit in the bar at the top, so you can also watch a collision frame by frame.
With the water tool you drag out a tank or a pool of liquid: the water flows, sloshes, spills over an edge and splashes. A block of cork floats on it, a block of steel sinks to the bottom, and a hollow hull carries a load until you stack it on one side. With the laser you place a beam of light that reflects off a mirror and refracts through glass; switch on colour separation and white light fans out into a rainbow inside a prism. And with the rocket motor you give an object a push along its own axis, on and off with an arrow key. On top of that you can couple two axles with a gear, hang a load from a chain or a rope, glue two parts together, let a point draw its path with the trail, and turn an object into a centre of gravity for the rest to orbit.
Behind the menu button are fifty-one ready-made scenes, spread over ten folders: water (floating and sinking, a dam break, a tank draining through a hole, a water wheel, a ship that capsizes), light (a prism, a glass ball with a focal point, a hall of mirrors, the critical angle), drivable vehicles (a car on the arrow keys, a truck with a trailer, a monster truck with real suspension, a tank with a turning turret and a gun), structures (a gear train, a gear clock, a crane, a chain bridge, a cable car), force and wind (a rocket, a moon lander, air resistance, a wind tunnel, a sailing boat, planets around a centre of gravity), measuring and graphs, editing and time, games and challenges. Every example shows one thing and starts running straight away; one line at the bottom says what there is to do. They are ordinary scenes: you may change anything, and what you make you save as a file on your own device.
Under the canvas runs a rigid-body engine: every object has a mass, a centre of gravity and a moment of inertia, and each step the engine resolves the collisions, the friction and the stacking. That is the same kind of arithmetic that sits inside 2D games. What is not in it is deformation: a block does not bend and a ball is not squashed. The water is not a solid block but a cloud of particles that correct their mutual spacing every step, and the light is not a wave but a ray that is reflected or refracted at each surface — enough for a lens and a prism, not for diffraction or interference. Everything is computed in metres, kilograms and seconds, with a fixed time step of one sixtieth of a second — and that last part is no detail, because an engine that keeps pace with the frame rate gives a different outcome on a fast monitor than on a slow one. Gravity is set to 9.81 m/s², so what you see falling really does fall at the rate you know from outside.
The sandbox has a steering head (turn an object into something that drives by itself and turns on two keys), a trail that becomes a wall or a deadly line, a counting gate that counts what passes through, a destroyer that removes whatever it touches, and a scoreboard where you set who plays, which keys they have, what earns points and when the game is won. With that you build a game without a single line of code: the Games folder holds Achtung, die Kurve with six players on one keyboard, a snake game running on the same steering head, and a race with lap counting where two cars drive around turning points. In the Challenges folder every scene comes with an assignment: drive a car over a course past the lava, get a marble into the bin through a chain reaction, knock a tower down with a catapult, turn three mirrors until the laser beam hits the target, lift a boat through a lock, or put a crate on a pedestal with a crane.
For a lesson this is a way to show centre of gravity, stability and collision without anything getting broken — and just as well Archimedes' principle, the refraction of light and the projectile path: build a tower and roll a ball into it, drop a block of cork next to a block of steel into the water, or fire three projectiles at 30, 45 and 60 degrees and let the class guess first where they will land. For anyone designing a mechanism it is a scratchpad — a ramp, a seesaw, a gear train, and within a second you can see whether the idea holds up. And for everyone else it is simply fun to knock blocks over. Nothing goes to a server: the simulation runs entirely in your own browser, you do not have to install anything, and no scene is kept that you do not keep yourself.