Why do the lemon pips continually rise up and fall down in my gin and tonic?

Geoff McKay of Kingston, Ont., asks: "Why do the lemon pips continually rise up and fall down in my gin and tonic?"

Dr. Kathy Focsaneanu, an assistant professor in the Department of Chemistry and Biomolecular Sciences at the University of Ottawa, says the rough, irregular surface of the seed provides a "nucleation site" for the formation of a small bubbles of carbon dioxide gas.

It's much easier for dissolved gases to bubble out of the liquid phase at small irregularities or discontinuities than on a smooth surface, like the inside of the glass.

As more carbon dioxide molecules come out of solution, the bubble grows, acting as a flotation device for the seed, and the seed rises. 

Eventually, the bubble dislodges from the seed. Without the seed's "life jacket," the seed falls, and the process begins anew. Focsaneanu says the cycle continues until the beverage goes "flat" — meaning, no more dissolved carbon dioxide is present. Or until you finish your drink.