Observer effect (physics)

The observer effect is a scientific concept that basically says that when you measure or observe something in science, the results of the measurement can be changed by the act of using special tools to measure it. This can be a little confusing to think about, but here is an example you can use to help. Say you want to know how much water is in a pond. If you just look at the pond, you might be able to estimate the amount of water in the pond, but you won't know for sure. If you use a special tool like a measuring cup to measure the exact amount of water in the pond, then that tool has changed the water in the pond and the measurement would no longer be the same as it was before you measured it. This is the observer effect in physics.