As with any scrumptious cocktail, the proof's in the amount of alcohol involved. Now, some places serve their mojitos weak. So weak, in fact, that they taste like lemonade with mint leaves thrown in. Not so at Sierra Bonita, the excellent Southwestern restaurant that's into its second year in Phoenix. At Sierra, the bartenders know how to mix the restaurant's specialty. So much so that ? try as we may this year ? we couldn't (during an exhaustive survey of bars in the metropolitan area) find a mojito with as much kick. Hic! For those of you who haven't an inkling of what a mojito's all about, let us school your ass in how one's mixed: Squeeze a lime into a bowl. Add a tablespoon of sugar and a bunch of washed mint leaves. Mush up the mess really well and pour it into a tall or fat glass of crushed ice. Fill the glass to about three-quarters of an inch from the top with good white rum. Sierra Bonita uses Bacardi Lim?n. Top the concoction with club soda or Sprite, stir thoroughly, and drink. It's the perfect treat for a hot Phoenix night. In fact, the night we were last at Sierra Bonita, it was 100 degrees after dark, and while at first we minded being seated on the outdoor patio (the beautiful indoor bar was packed), we soon didn't mind at all. After our third $6 mojito, we may as well have been in La Jolla.