Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.

Sentence1: if fossils of an animal or plant are found in a place then that animal or plant used to live in that place. 
Sentence2: Fossils are preserved original remains or preserved evidence of plants and animals.
fossils

Sentence1: Eggs are usually laid in a nest. 
Sentence2: birds have nests.
nest

Sentence1: tadpole changes into a frog. 
Sentence2: tadpoles become neat and cool.

changes