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.

Sentence1: Amphibians are the first true tetrapods, or vertebrates with four limbs. 
Sentence2: Frogs are classified as amphibians.
Amphibians

Sentence1: measuring the hardness of minerals requires scratching those materials. 
Sentence2: To figure out the hardness of a mineral, the Moh's scale of hardness is used.
minerals

Sentence1: condensation causes clouds to form. 
Sentence2: With increasing cloud blocking the sun, temperatures drop considerably.

clouds