Here's a trick related to excretory products:

1. Materials needed: A clear plastic cup, water, and a straw.
2. Fill the plastic cup halfway with water.
3. Secretly place a small amount of food coloring or a drink mix powder in the bottom of the cup, hidden from view.
4. Hold the cup in your hand and ask someone to choose a colored straw from a selection of different colored straws.
5. Once they choose a straw, instruct them to blow into the cup through the straw, creating bubbles in the water.
6. As they blow into the cup, the water will mix with the hidden color, making it appear like the excretory product of their chosen color is being produced.
7. Act surprised and tell them that they just created their excretory product with their magical straw!

Remember to always be respectful and considerate when performing tricks related to bodily functions, as it may not be suitable for all audiences.