How does a grenade work?

Pull the pin, release the lever and after a few moments, it explodes. From what I think I know, it’s made of a high carbon metal that shreds easily when it goes off sending shrapnel everywhere. What causes it to delay the explosion? What kind of moving parts are in there, if any? And are they the same kind of grenades the troops have been tossing since WW1 or have there been many technological advances in the science of grenadology?
-M

How grenades work.

Pretty good, thanks.
(But I could definatly do without the Internet Explorer Script Errors and the pop-ups everytime I go to the next page. That’s why I try to stay away from that site.)
-M