Bounce

roll, jump, thread every hoop. grow to float, shrink to fit.

roll a ball through hoops and spikes; grow to float on water, shrink to fit through gaps

The ball carries momentum, so where it lands depends on what you did a moment ago. Growing lets it sit on water; shrinking is the only way through a narrow gap. Every obstacle asks which size to be.

Concepts: elasticity, constraints.

An interactive you play with one thumb, on Wundr.