A plastic heart rests on the water and doesn't tip over, no matter how you turn it. It freezes in any position, as if the water were holding it by invisible strings. This trick is possible thanks to the Zindler curve — a shape in which all lines that divide the area in half have the same length.
Scientists crafted this heart from several layers of material, achieving a density exactly half that of water. In this case, gravity and buoyancy balance each other at any tilt. If the density is slightly altered, the body begins to prefer certain orientations, like a boat that always turns its bow into the wave.
The experiment shows how geometry governs buoyancy. Even small inhomogeneities in the material or capillary effects at the water's edge can disrupt the perfect equilibrium. Such simple experiments help test mathematical theorems in practice and could be useful in designing floating structures.
The Zindler curve is named after the Austrian mathematician Konrad Zindler, who described it in 1921. The heart is just a special case; there are infinitely many such curves.