Bell's theorem

Bell's theorem tells us that even if we can't measure something, it still affects what we are able to measure. Imagine that you have two boxes. Each box has something inside of it that you can't see. You want to know what is in the boxes, so you try to measure it using light. Light bounces off the boxes and tells you what is inside. Bell's theorem says that even if you can't see inside the boxes, the things inside still affect how the light bounces off of the boxes and the way you measure them.