Subject: how does the human brain judge time?

How does the brain judge time?  I am doing a science project and I have picked 
the problem:  Does Activity affect the way we judge time?  For example, we are 
going to have a person just sit there, doing nothing, and time that person.  We 
are then going to ask them when they think 5 minutes is up?  With that same 
person, we are going to have them do a task and ask them when 5 minutes is up?  
We're kind of trying to see if time really does fly when you're having fun.  But 
how do we judge time at all?  

Re: how does the human brain judge time?