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.
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.
Underneath 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 solves the collisions, the friction and the stacking. That is the same kind of computation that sits inside a 2D game. What is not in it is deformation: a block does not bend and a ball is not squashed. Everything is calculated 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 follows the frame rate gives a different result 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 the world outside.
For a lesson this is a way to show centre of gravity, stability and collision without anything getting broken: build a tower, roll a ball into it, and let the class guess first. For anyone designing a mechanism it is a scratch pad — a slope, a seesaw, a stack, 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 stored that you do not store yourself.