How does the lemon juice work as invisible ink?
A CHEMICAL REACTION! That is right, in your kitchen or classroom a visible chemical reaction is happening. This reaction is between oxygen and the molecules in the lemon juice. That is what causes the brown color. This is not a reversible reaction which means that once your writing has become visible it cannot become invisible again. The ladies over at Layers of Learning have some great information on oxidation.
Extend the fun:
There are lots of great chemical reactions extensions that you can do including the infamous volcano. One of my kids favorites is to take a water bottle and add a couple of teaspoons of baking soda (you can add more as you need or want) to about 1/2 cup of vinegar and cover with a balloon. You will need to be fast, but the reaction results in the release of carbon dioxide that actually blows up the balloon. This can also be used to explain that gas takes up space (the carbon dioxide has filled the balloon). I love science!
If you want something a little less messy you can create art with black tea and lemon juice. The black tea can be used as paint and the lemon juice will chemically bleach out the tea. You can create patterns or writing. This is found in numerous places, but there are some more thorough instructions from HubPages.
If you want to talk more about oxidation and fruit. You might try this experiment from the Indianapolis Marion County Public Library kids blog.
Ask your child if they can think of a purpose for invisible ink? I am sure they can create a pretty interesting story. You might offer to take it down for them and let them illustrate it (maybe even with invisible ink).
Let your child see the joy in learning and they will want to learn.