Ferro

hold to magnetize. release to relax.

a ferrofluid droplet — spikes under your touch

Real ferrofluid does this — push a magnetic field past a threshold and surface tension loses its fight, so the liquid stands up in spikes, then relaxes back to a plain droplet once the field drops. Your finger stands in for the magnet here: hold it and the droplet bristles, let go and it settles.

Concepts: magnetism.

An interactive you play with one thumb, on Wundr.