Step 1: Get the bowl or container and squeeze half a lemon.
Step 2: Pour some water in, a tiny bit more then the lemon.
Step 3: Mix it around with the spoon.
Step 4: Dab the cotton bud in and start drawing (Dab for more, often).
Step 5: Let your drawing dry. Now put in the oven for 3-5 mins approx. on high heat. You can do this with a light bulb instead. Even in the sun it simply needs to be heated. (Check the oven often, the paper can burn)
Why does this happen? Lemon juice oxidises (combine chemically with oxygen) and turns brown when heated. Water diluted the lemon juice so that it wouldn't be noticeable on the paper until heated. This experiment can with work with vinegar or milk as well.
Make this a experiment The "experiment" above is actually a demonstration. To make it a real experiment try to answer questions.
1. Does any other liquid work 2. What do milk, lemon and vinegar have in common that makes it work