I first learned about this drink after hearing it mentioned on TV. Apparently, it's also known as a tequila fresca and is very popular in Mexico. Ole!
1 part tequila
6 parts Fresca (or other grapefruit soda)
Fill a tall glass with ice and add the tequila and soda. Serve.
Fun fact: Although I hate grapefuit (it is my least favorite food) and I'm not a big soda drinker, if I am going to drink soda, Fresca is one of my favorites. Weird, huh?