Spin

pull your arms in. the universe keeps the books.

drag a skater’s arms in and out — spin rate obeys a conserved law

Angular momentum, spin rate times how spread out the mass is, can't change on its own. Pull the arms in and the spin speeds up to keep that product constant, with no push added: the increase is the conservation law paying out. Collapsing stars spin up too.

Concepts: angular-momentum, physics.

An interactive you play with one thumb, on Wundr.