There's a reason everyone is still so crazy about squats: They're an all-in-one blast for your glutes, thighs, and core, meaning you're building muscle all over your body in one epic move. But there's no reason to stick to basic squats for the rest of your life. We tapped Alex Silver-Fagan, an ACE-certified Nike trainer, to round up all the best variations to create this 30-day squat challenge. Ho