What a strange device! But it sure is fun. When you grip the glass base, heat from your body warms the air in the outer tube. As air heats up, it expands. It may look like the liquid is boiling, but the expanding air is actually displacing the liquid up through the center of the glass device. As the liquid reaches the top, the warm air continues to rise, sending bubbles through the liquid. Voilá! The liquid looks like it’s boiling. The glass is very thin. Please supervise your little ones. 7-inches high.