One of the important ideas in science is the difference between an observation (what we can see, hear, smell, touch, taste, or measure directly) and an inference (the opinions we come up with based on our observations).

Scientists have to be very careful not to confuse the two things. For example, a door slams and your cat runs the other way. You can observe the cat running. You may infer that it ran away because it was startled or scared, but that's an opinion. Maybe the noise hurt its ears, maybe it's chasing a mouse in the next room, or maybe it was going to run anyway and the noise was a coincidence. If the cat were a person, you could ask why it ran, but people don't always tell the truth. So you could say "Katie says she ran because she thought she heard someone call her." But you can only OBSERVE that she SAID that. You can't observe what she was thinking when she ran.

Lots of people have tried to use science to come up with "lie detectors" like the polygraph. A polygraph basically graphs a few things about your body ("poly" means many) like you breathing, blood pressure, and sweating. These things usually increase when you are under stress. (This response is called the "fight or flight" response. Why would this response get us us ready to flee or fight?) The idea behind a polygraph is that when you lie, you feel stress, and the machine will measure that. But these machines can be "fooled" by people who know how. That's why polygraph test results are not allowed into court as evidence.

You could do an experiment to see how good you are at inferring what people are thinking based on your observations. You could have people tell you something, then write down whether it was true or not on a hidden piece of paper. Then guess at the answer. (Why would you have them write the answer before your guess?)

I like your question because I always want to know what people are really thinking (just kidding!).

Anyway, I want to remind you that science relies on the scientific method, which means that we can only test things that we can measure. There are medical instruments that can measure which part of your brain is active. Scientists and doctors can look at that and guess what mood you're in. If you are being analytical, one part of your brain is activated. The same is true if you are sad. However, this machine cannot tell what is making you sad, or why you are thinking so hard about something. Do you know what this machine is called? It's called and electroencephalogram, or EEG. Try looking it up in an encyclopedia for a picture! Wow, some of them are really big!