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.
--------
Question: Sentence1: if an object is black then that object absorbs all visible light.
Sentence2: Something is black if it absorbs visible light and doesn't reflect.

Answer: light

Question: Sentence1: Urine leaves the body through a sphincter in the process of urination.
Sentence2: urine leaves the body upon relaxation of a muscle.

Answer: Urine

Question: Sentence1: erosion causes a river to become deeper and wider.
Sentence2: On the contrary, the NILE is the longest river in Africa.

Answer:
river