Learn more physics!
How does a love meter work? It is a glass object with liquid in the bottom. A tube runs through the "meter" to the top. When you put your hand on the bottom of the meter, where the liquid is, the liquid rises through the tube to the top. Then the liquid starts to boil at the top.
The device you describe is quite simple to explain. Most "love meters" are made by putting a volatile liquid (like ether) inside the glass container that looks like two spheres connected by a skinny tube. (Volatile simply means it evaporates easily).
When you grab one of the glass spheres, the warmth of your hand heats the liquid inside causing more of it to evaporate (that is, turn from a liquid to a gas). A gas occupies much more room than a liquid, which means that the pressure in the held sphere will increase. This pressure pushes the liquid out of the sphere that you are holding into the other sphere.
The movement of the liquid from one sphere to the other can often be very quick, causing it to "spout", making it look a bit like boiling water.
Hope this helps.
(published on 10/22/2007)
Follow-up on this answer.