Sled

draw a track. see how high the sled can climb back.

draw a track in pencil and a sled runs it: it can never climb back above the line it started on

A sled has no engine, so its speed anywhere was bought by falling: how far below its starting height it is. A hump above that line can't be climbed, because the run-up can't give back more than it took.

Concepts: energy, motion, conservation.

An interactive you play with one thumb, on Wundr.