Use a friction erasable pen and normal pen to make a message on a piece of paper which changes when the paper heated up. Useful as a fun demonstration when discussing that work done against or by friction causes energy to transfer into the thermal store. Make sure you are careful when heating the paper to not set it on fire or burn yourself. Good for teaching physics at GCSE.

Thanks to @thesciencebehindmagic for the original idea:
   / @thesciencebehindmagic