Writing secret messages. You can do writing that appears invisible-yet is easily revealed-with lemon or onion juice. Use either bottled or freshly squeezed lemon juice. To get onion juice, grate a peeled onion into a dish, then strain out the juice. Let it set a while before using.
Dip a toothpick in the juice and write on hard-surfaced paper. Let the juice dry thoroughly. To make the message appear, hold the paper next to a hot lightbulb or an electric iron. Don’t let the paper catch fire.
You can also do invisible writing with milk. Use a toothpick and allow the milk to dry. To expose the message, rub pencil shavings or fine ashes lightly over the surface.