It's a game of balance between liquid & vapor. They want to be equal. So, What's the invisible force pushing the liquid? Vapor pressure. Heat from your hand transfers energy making the vapors go wild. As it gets warmer, those vapors get feisty and force the liquid up to the top bulb. The liquid inside doesn't actually boil. Because there's a temperature difference between the two bulbs, the liquid moves from high pressure to low pressure. As the gas expands, it bubbles through the liquid, making it look like it's boiling.