A few pieces of paper Cotton buds to write Solutions like sugar solution, fruit juice, salt solution A pen to note different solutions used A toaster to heat and read messages
Take the cotton buds and dip each of them in a separate solution (sugar solution, fruit juice, salt solution) and write the messages on separate pieces of paper. The writing will be invisible. Note down on one side of the sheet the solution used to write invisibly. Allow the writing on the paper pieces to dry for some time. Turn the toaster on and hold the paper with messages over the heat in the toaster. Stop when the paper turns brownish, but not burnt. See if you can read the messages. If yes, send secret messages to your friends by invisible writing method.
Heat helps to make sugar into toffee, and fruit juice into a compound of sugar and protein that is contained in them revealing as a visible message. Invisible writing is seen due to a compound being formed on exposure to heat.
* You will be able to read what you wrote with the invisible ink, sugar and fruit juices being better than the salt solution.