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: friction causes an object to lose energy. 
Sentence2: Friction occurs when surfaces rub together.
[A]: friction


[Q]: Sentence1: Similarly, stars are referred to as suns, but our star is the Sun. 
Sentence2: our star causes water to evaporate more quickly by adding heat.
[A]: our


[Q]: Sentence1: Male gametes are sperm. 
Sentence2: fertilization of sperm produces a diploid zygote.
[A]:
sperm