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: Generally, pheromones are volatile substances. 
Sentence2: Male dogs use volatile substances in urine to mark their territory.

A: substances
****
Q: Sentence1: pathogens can cause disease. 
Sentence2: pathogens affects the immune system.

A: pathogens
****
Q: Sentence1: biofuel is used to produce electricity by burning. 
Sentence2: Ethanol can produce electricity by burning.

A:
burning
****