The spring from a pen at home contains elastic potential energy to write with.

Click pens

The click pen works because of the spring. 

The spring has created tension with the ballpoint of the pen, which permits it to stay exposed. When the end of the pen is clicked again, the tension is released causing the ballpoint to retract into the frame.

Pen (Spring)

When the top of a pen is clicked down, a spring is compressed (storing elastic potential energy) so that when it is clicked again it can retract the tip inti itself.