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
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Q: Sentence1: Roundworms make up the phylum Nematoda. 
Sentence2: Also, nematoda and rotifera are important groups contributing to soils.

A: Nematoda
****
Q: Sentence1: If fertilized, the egg implants itself in the uterus. 
Sentence2: Sperm had to swim from male to female reproductive organs to be implanted in the uterus.

A:
implants
****