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.

Q: Sentence1: Bird eggs have hard, calcium carbonate shells. 
Sentence2: The reproductive cells of a bird is a hard, calcium carbonate shell.

A: calcium
****
Q: Sentence1: wind is used for producing electricity. 
Sentence2: Winds are caused by pressure differences.

A: wind
****
Q: Sentence1: Lancelets are small fishlike animals with tapered bodies. 
Sentence2: small fishlike animals with tapered bodies are free-swimming.

A:
animals
****