An insect larva builds a sand pit engineered so nothing can climb out

Antlion larvae dig steep conical pits in loose sand, spiraling backward while flinging grains with their heads until the walls sit right at the angle where sand barely holds together. Any ant that stumbles in triggers a miniature landslide toward the antlion's jaws waiting at the bottom, and if it tries to climb out, the larva flicks sand to collapse the slope again.

— Jeffrey R. Lucas, The Biophysics of Pit Construction by Antlion Larvae — Animal Behaviour, 1982