Invisible ink

Materials

white paper

white vinegar, lemon or lime juice

cotton buds

heat source: hair dryer, lamp or toaster

Difficulty:

Time: depends on drying time

Adult supervision: possibly to help with revealing the message

Dangers: heat

Put some lemon juice or white vinegar in a small bowl/container
Use the cotton bud as your pen: dip the tip into the lemon juice or vinegar and write a message on the paper. Make sure the "ink" penetrates the paper.
Wait for the paper to dry
To reveal the message, you may need the help of an adult. I got good results with a hair dryer. Alternatively, turn your toaster to the lowest heat setting and carefully hold the paper over the heat. Some methods suggest holding the paper up to a high wattage lamp.

The heat reacts with the lemon or lime juice or vinegar in a process called oxidation turning it brown.