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.

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!