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: a mutation in the sex cells of a parent can cause a new trait to appear in the parent 's offspring. 
Sentence2: Traits are initiated by mutation and recombination.
[A]: mutation


[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: a sandbar is formed by water moving sediment downstream. 
Sentence2: a sandbar is formed by natural erosion.
[A]:
formed