Subject: How does a see-saw really work?

Yes, I understand that when force is applied to one end, it impacts the 
other and I also am aware of the concepts of a fulcrum.  However, in more 
detail, how and why does a see-saw move?  And more specifically, what rules 
or constraints are involved to make certain motions impossible on it?  Is 
there a formula to predict where it will be at any time?  

This isn't for school so you can take your time answering it.  I'm just 
trying to expand my knowledge :)  Thanks.

Re: How does a see-saw really work?