Lego axle to motor connector

I wanted to mix two kits and create an interesting science experiment for my son. We have a bunch of legos and we have a solar kit that came with a motor. The solar kit already shows how the motor rotates when it's connected to an electricity source. We also did "beaker blender" from a lego contraption kit that shows how to use gears and increase the rotation.

I wanted to show that by rotating the motor, it produces electricity. However, to do this I needed a way to connect the lego axle with