Today we’ll McGeiver some invisible ink using nothing but staples, white vinegar and a damp teabag. Most people already know about the lemon juice/heater method but with this one you won’t need to burn your paper to reveal the message.
Get about a quarter of a glass of white vinegar and drop in a load of staples. Paperclips, nails or any other non galvanised iron or steel object will work too but staples being small will have more surface area to react with the vinegar. The acetic acid in the vinegar will react with the iron to make iron acetate, the key ingredient for the ink.
Leave this to sit for at least a day. When complete, pour off your vinegar/iron acetate solution into a clean jar and throw away the manky staples. Using a pen of sorts you can now write with this ink.
Revealing the message is as simple as rubbing over it with a warm damp teabag. The iron acetate will slowly turn dark and your message (or crudely drawn genitalia) will show like magic.