Why do lava lamps always leave that “nipple” of lava floating at the top? What is so special about that portion?
Nothing “special”, per se. I would imagine it’s just the leftover scrap of the waxy “lava” that was held on the surface by surface tension and isn’t quite heavy enough to break surface tension and fall to the bottom.
When the lamp is turned off and starts to cool, the lava starts to settle. There’s usually some at the surface of the liquid. Most of the surface blob will break away and sink, but there’s always that tiny bit left that can’t break free. Anyway, it’s the breaking away of the larger lump that creates the “nipple”.
If it offends you, a gentle shake (and I do mean “gentle”, just rock it back and forth a couple of times) after the lamp has cooled will help the “nipple” break through the surface tension and fall to the bottom.