When I was a kid, my friend had this pen with invisible ink. You could ... ink to show up you need a spray bottle with iodine tincture. That iodine turns the paper purple but the lemon juice ...

While it is permanent, it's also washable. We'll take invisible ink scribbles on our walls over purple Sharpie any day! And stocking stuffers should ideally cost below $20, so at just $8 for a six ...