Question

What happens when you hold the two ends of a spring and pull them apart?

Answer 1

When you pull them apart..you are using your energy in it and that energy is converting in potential energy of that spring.(according to conservation of energy)
then after you leave it...it tries to retain its original shape with the help of that potential energy...
By the way that energy is equal to
1/2KX^2

where k is spring constant
and X is how much you have stretched the spring

Thanks

Answer 2

When you pull string apart from each other the energy is stored in the form of potential energy. The string gets stretched to it's limit and then when you leave it ,potential energy is ultimately converted into kinetic energy. That is why it hurts when you leave it!