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: pathogens can cause disease. 
Sentence2: pathogens affects the immune system.

A: pathogens
****
Q: Sentence1: when a hemisphere is tilted away from the sun , that hemisphere receives less direct sunlight. 
Sentence2: Sunlight is delivered in small packets of energy called photons.

A: sunlight
****
Q: Sentence1: skin is used for breathing air by frogs. 
Sentence2: skin is used for breathing air by toads.

A:
air
****