Overhead extensions aka skull crushers tone your triceps. 

Remember to keep your elbows close together (on either side of your head). It is best to do this move lying on a bench but if you do not have a bench, you may do this exercise on the floor.

Starting Position
Hold a dumbbell in each hand and lie on a flat bench with the feet flat on the floor. Extend the arms up so that the dumbbells are directly above the shoulders and the palms are facing each other. Keep the abdominals tight and the lower back in a neutral position.

Bending the elbows, lower the dumbbells to the sides of your head. Keeping the elbows in place, extend your arms up to the starting position, pause and repeat.