To do the experiment you need:

• 2,5% or fat-free milk
• A dish for the milk
• 2 brushes
• A white sheet of paper
• Finely ground pepper (in a pepperpot)

1. Pour the milk into the dish. Take a clean brush, steep it into the milk and write a secret message.

2. Wait till the message get dry. Then we will make it visible again: 

sift the ground pepper  and with the second brush distribute it on the sheet of paper. The mysterious pattern is seen again.

Hurray! We revealed the secret.

Explanation of the experiment:

 Milk, which is  in water (88 %) is absorbed into the paper, but unnecessary fat, carbohydrates and protein adhesive, especially casein, remains on the surface. When we sift some ground pepper on the sheet of paper, the grains stick on the protein of the pattern and the mystery message  becomes  visible again.