A simple pendulum has a tough time changing energy between two types: kinetic and potential. This is because it deals with friction and air resistance.

1. Potential Energy (PE): When the pendulum is at its highest point, all of its energy is potential. The formula for this is:
   $PE = mgh$

2. Kinetic Energy (KE): When the pendulum swings down to its lowest point, all its energy is kinetic. The formula for this is:
   $KE = \frac{1}{2}mv^2$

To help reduce energy loss, you can use lubricants and try doing experiments in a vacuum where there’s no air.