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: ultraviolet light causes sunburn. 
Sentence2: Sunburn risk is higher at elevation.
sunburn

Sentence1: Female monotremes lack a uterus and vagina. 
Sentence2: Babies are normally delivered from the uterus through the vagina.
uterus

Sentence1: Plants are multicellular eukaryotes with cell walls made of cellulose. 
Sentence2: Plants cell walls are made of fiber.

Plants