How is Coca-Cola a soda?
The name Coke is a nickname for Coca Cola.
A long, long time ago, Coca Cola contained Cocaine in its
ingredients, seriously! Cocaine wasn't always outlawed in the US,
you know. In fact, over the counter medications would carry some
(small) level of Cocaine in their ingredients and doctors would
prescribe higher doses of pure Cocaine for such illnesses as
fatigue.
Then Cocaine was outlawed, and Coca Cola took it from its
ingredients.
The history should give you a hint at the names: Coca Cola and
Coke for the popular soda.