An ice cream soda or ice cream float (called a spider in Australia and New Zealand) is made by adding soda pop or seltzer to ice cream. Some people add flavoring, like chocolate syrup, or a little milk. However you make it, as soon as the soda hits the ice cream you get fizzy, frothy, tasty bubbles.
Do you know how it works? It's basically the same as what is going on with the Mentos and Soda Fountain, except not as messy.
You are knocking the carbon dioxide in the soda out of solution. Bubbles of air in the ice cream provide nucleation sites around which carbon dioxide bubbles can form and grow. Some ingredients in the ice cream lower the surface tension of the soda so the gas bubbles can expand, while other ingredients trap the bubbles in much the same way as small amounts of protein in seawater trap air to form seafoam.
I like all types of floats, including black cows (coke floats with cola and vanilla ice cream), brown cows (root beer float with root beer and vanilla ice cream), and purple cows (grape soda and vanilla ice cream), but you can use other ingredients. Here's a recipe for a coffee cola Float, which is bubbly and caffeinated and therefore a double-win:
2-1/2 cups coffee (room temperature or chilled)
2/3 cup light cream or milk
coffee, chocolate or vanilla ice cream
Mix the coffee and cream or milk, pour it into glasses, add scoops of ice cream, and top it off with cola.
You can garnish it with whipped cream, chocolate covered coffee beans, or a little coffee powder or cocoa.