Simplify the following text for a Grade 7 native English-speaking person.
Original: They are usually made of some organic material such as twigs , grass , and leaves ; or may simply be a depression in the ground , or a hole in a tree , rock or building .
Simplified: Many times they are made of things like twigs , grass , mud , and leaves ; or they may simply be a low place in the ground , or a hole in a tree , rock , or building .