Anticopyright.com: Intellectual "property" does not behave like material property. If I give you a physical object I may no longer have use or control of that thing, and may ask for something in return -- some payment or barter. But when I give you an idea, I lose nothing. I can still use that idea as I wish. I need ask nothing in return.

If I have one cookie and you have one cookie, and we exchange that, we still have one cookie each. If I have one idea and you have one idea, and we exchange that, we have two ideas. -- Bernard Shaw

This quote tells us the exact difference between intellectual and physical property.