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: a rainbow is formed by refraction of light by separating light into all different colors. 
Sentence2: Rainbows form when raindrops refract sunlight.

A: refraction
****
Q: Sentence1: After a seed is planted it is watered. 
Sentence2: seeds need water.

A: seed
****
Q: Sentence1: Tires leave skid marks and sometimes smoke. 
Sentence2: braking can leave marks.

A:
marks
****