Two years ago my class went on a school trip in Maribor, Slovenia. We were visiting "Hiša eksperimenatov" which means "The house of experiments". My friend Lucija was really impressed with that place (even thought she didn't understand everything that was going on haha). In order to prank her, I took a bottle, filled with water, in which there was a little corck, and if you squeesed the bottle, corck would sink to the bottom. My friend hadn't seen that bottle yet and I told her that she should see this trick: "If you pull this invisible string underneath the bottle (which, of course, didn't exist), you will pull this corck to the bottom". I sqeesed the bottle and pulled the "invisible string" and the corck sinked down. She was extremely impressed and immediately took the bottle to our teachers (one of them was actually our physics teacher) trying to show them that incredible trick. She pulled and pulled the "string" but the corck didn't sink to the bottom so she was really confused. Being quite humiliated, she didn't want to come across as dumb so she started pulling the string more energetically but it didn't help. Finally, one of the teachers finished her agony by telling her that she knows the corck sinkes down if you squeese the bottle. At that point Lucija's brain finally started to work and when she figured it all out, Lucija looked at me, all confused and angry and she loudly yelled my name. I was watching her the whole time and was laughing to her naivety. Even though she got quite embarrassed due to mine meanness, Lucija actually learned a valid point that she can't trust her friends so easily.