The observer effect: where the act of observing something changes it. Sounds like a great way to get out of a speeding ticket, right?
But in all seriousness, it's a real thing. In quantum mechanics, the act of observation can cause a wave function to collapse, effectively changing the state of a system.
It's like when you're at a restaurant and you see a dish you really want to order. You order it, and suddenly it's gone. But if you hadn't seen it, you wouldn't have wanted it, and it would still be there. See?
Or, you know, maybe it's just like when you're at a coffee shop, and you see a pastry you really want to buy. You buy it, and suddenly it's not as sweet as you thought.
In any case, the observer effect is all about how our perception changes the world around us.
Want to see some examples?