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.

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

[EX Q]: Sentence1: bacteria cause food to spoil. 
Sentence2: Food poisoning occurs when spoiled food is eaten.
[EX A]: spoil

[EX Q]: Sentence1: BTW, a gamete is like an egg or sperm. 
Sentence2: Adult sponges produce gametes.
[EX A]:
gamete