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: lightning can cause a forest fire. 
Sentence2: Lightning can cause a natural cycle in forests.
[A]: lightning


[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: an animal requires warmth for survival. 
Sentence2: Cubs are born in the winter and nestle in their mother's fur for warmth and to nurse.
[A]:
warmth