why it's "on top of sth" instead of " on the top of sth"?
I looked a word "lap" up in the dictionary. The definition is " If you have something on your lap when you are sitting down, it is on top of your legs and near to your body."
I'm wondering why it's " on top of your legs" instead of " on the top of you legs"
thanks for your help.

Question

Answer

It cannot be "on the top" because your legs don't have a top! In order to be on the top of something, that thing has to have a top!
on the top of that pile
Otherwise you say:
1. on top of
2. AT the top of
at the top of the class
at the top of the mountain

Answer

“On top" is an expression which means:
1.	 At the highest point or peak.
2.	In a dominant, controlling or successful position.
	
“On top of” is an expression meaning:
1.	On or at the uppermost point or side of.
2.	Informal. 
         a.	In control of
         b.	Fully informed about
3.	Besides; in addition to
4.	Following closely upon; coming immediately after.

A good dictionary, such as the American Heritage Dictionary of the English language (if you can afford it) would answer questions such as this one.