In kindergarten, I put a strip of paper that said "candy" on a mini Tabasco sauce bottle and gave it to my teacher. She just smiled and laughed in amusement. I didn't actually want her to get hurt so I didn't mind her not drinking it.
My sister used to eat cereal everyday for breakfast so I would take her box of her favorite kind and switch it with my dogs food, which looked like Resess. Get it?! She thought the cereal was out of date...but really she ate dog food! (You can also find some good prank videos on youtube.)