The wall sit is a bit different from typical squats since you're holding a static position for a certain period of time, rather than working through an entire range of motion. This is a great exercise you can do anywhere without any equipment to help you build endurance in the lower body. Here's how to do it:
1. Stand in front of a wall (about 2 feet in front of it) and lean against it.
2. Slide down until your knees are at about 90-degree angles and hold, keeping the abs contracted, for 20-60 seconds.
3. Come back to start and repeat, holding the squat at different angles to work the lower body in different ways.
4. To add intensity, hold weights or squeeze a ball between the knees.