You can do this exercise either at the gym or at home. If you prefer to do them at home, you can either purchase free weights or use item like water bottles or soup cans.

This exercise works your triceps, which is the back part of your upper arm.c 

1. With your weight in your right hand, raise your arm above your head and bend at the elbow as if you were trying to scratch your upper back.
2. Make sure your elbow stays pointed at the ceiling and remains close to your head. If you need help stabilizing your right arm, reach across your face with your left arm and hold your elbow to your head.
3. Extend your right arm so that you are lifting the weight towards the ceiling.
4. Lower back down to complete one rep.
5. Perform 15-20 reps with each arm for one set. Complete 3 sets.