Thoughts fly in and out of your head all the time.
coughEDWARDCULLENcough It's quite possible for another person to be thinking the same thing as you (you might notice that whole "Hey, no way! That's what I was thinking!!!"), but most of the time, that doesn't really happen. You may be so absorbed in your thoughts that you're in your own little world. Whether they're good or bad thoughts, well, that's really up to you.
You'd think the mind of a small child watching TV would differ from the mind of a man who has just committed a crime. Or, the thoughts running through a mom's head holding a new baby, as opposed to the ones of a girl who just broke up with her boyfriend. What might one think before taking a test, or after a great day. What one elderly man would think before he dies, set against to a wife that just lost her husband. The thoughts you have when you wake up to the alarm clock in the morning, and the ones right before you fall asleep.
It might be easy to be lost in your thoughts, or even have one thought that never leaves your mind. Try not to think too much ;D