What you need
* Half a lemon
* Bowl
* Cotton bud
* White paper
* Iron (this heat source gives the best results, so check with an adult first before you start!)
What to do
* Squeeze some lemon juice into the bowl. [Onion juice also works really well. Chop half an onion and place in blender with a little water. Spoon some onion mix into a small plastic bag with a hole in one corner, and squeeze out the juice.]
* Dip the cotton bud into the juice and write a message onto the paper. For finer lettering, try using tooth picks.
* Wait for the juice to dry, so it becomes almost invisible.
* When you are ready to read your secret message or show it to someone else, use a hot iron (with adult assistance) to iron over the writing.
What's happening?
Lemon juice is an organic substance that oxidises and turns brown when heated.
The juice will be almost invisible when you apply it to paper, especially when you use small, fine lettering.
The message will reveal itself when heated.
Other substances which work in the same way include orange juice, honey, milk and vinegar.
Got some lemons spare?
You'll need a cool drink after all that detective work you've just done.
If you have three or four spare lemons, squeeze the juice of them into a jug and add water and sweeten with either honey or sugar.
Adjust amounts of each to suit your taste, pour into your glasses, add ice and enjoy!
