The Color-Changing Martini
This drink smokes elegantly, changes from a calm blue to fuschia as it cools from room temperature to drinkably cold, and tastes like a dirty martini.
Gin (or vodka, for a vodka martini) ~ Vermouth ~ Dry Ice ~ Baking Soda ~ Red Cabbage
Red #$@%! Cabbage?
Yes. Red cabbage contains a water-soluble anthocyanin that is a pH indicator. At low pH (acid), it's red. It's purple at neutral pH, and goes blue then green as the solution becomes alkaline. You're not going to see the full range here, because we want the drink to be, well, drinkable.
To get the indicator, chop up a cabbage leaf, put it in a bowl with some water, and microwave until it's boiling (or just add boiling water and allow to steep). A purple pigment will stain the water.
Add a teaspoon of cabbage juice... I mean indicator solution - to the martini glass. Then add very small quantities of baking soda, just enough to turn the solution blue. Add gin (or vodka) and vermouth (~6:1) to the glass. You should have a pale blue clear liquid
Give the recipient the drink. Tell them to watch closely, then add a chunk of dry ice. It will sink to the bottom of the drink, and bubble away happily, slowly cooling the drink. It will also neutralise the baking soda and change the colour of the martini. See it in action in the video below: