Mote

every push is paid in mass.

a mote in a dark fluid that moves only by throwing part of itself: eat what is cool, avoid what is warm, and every push makes you smaller

A body floating in a fluid has no floor to push against, so the only way to move is to throw some of itself the other way. Momentum is conserved: the droplet carries mass one way and you go the other, and the faster you want to go the more of yourself you have to spend. Here the bodies around you are tinted by size relative to you, cool if you could absorb them and warm if they would absorb you, and every push you make changes that arithmetic a little. The game is deciding when a move is worth what it costs.

Concepts: momentum, conservation, physics.

An interactive you play with one thumb, on Wundr.