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: Amphibians have permeable skin that easily absorbs substances from the environment. 
Sentence2: Some substances are harmful to the body and block biological systems.

A: substances
****
Q: Sentence1: propulsion is used for flying by a spacecraft. 
Sentence2: When Apollo 12 took off, the spacecraft was hit by lightning.

A: spacecraft
****
Q: Sentence1: using renewable resources to generate power reduces greenhouse gases. 
Sentence2: Nuclear power plants reduce greenhouse gases.

A:
gases
****