The Bobbing Bird

L gave me this as a present some time ago but I have been saving it as an indoor blip for a wet day. I had told L about my childhood memories of these little toys in the dentist's waiting room and she tracked one down for me as a gift. They have two glass bulbs, connected by a glass tube. Inside is a liquid with a low boiling point. The bulb at the bird's head is covered in felt and the whole body pivots around the bird's legs. When the felt on the head is wet, the water evaporates, cooling the inner bulb. This condenses the gaseous form of the chemical inside, which reduces the pressure in the top bulb. The higher pressure in the lower bulb pushes the liquid up the glass tube and into the head. Eventually the head gets too heavy and tips forward. This tips the bottom bulb so the lower end of the tube is no longer completely in the liquid. This releases the pressure in the tube and the liquid in the head rushes back into the body and the bird bobs back into an upright position. If the head has bobbed into a glass of water, this will wet the felt and start the process off again. All very pleasing to watch.