My son loves this

The spy pen is something my son begged for at Christmas last year and that was the first I'd heard of it. It writes in invisible ink and the light on the cap shows what has been written. My boy thinks it's the coolest thing ever. I bought ours at MasterMind Toys for something like $7(ish) - I think? But I just found it on Indigo for about $5.

Great novelty gift idea, just a little pricy for what you get. I'd still buy it again.