It all depends on how you see things. If you want to create an idea on any field then you should take a look on the things which affect to this field and the things which are indirectly related to it. like if you want to make a prank for your friend or relative, then you should think of some new, never happened or unknown for your friends of relatives. You should search for the things which he hates and scare of or loves and excite him or her. Then you should search for the concept. You can search for youtube or google for prank ideas.