TASK DEFINITION: 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.
PROBLEM: Sentence1: If there is sun, there is sunlight, there is day. 
Sentence2: Energy enters ecosystems from the sun.

SOLUTION: sun

PROBLEM: Sentence1: Animals behave in a variety of ways which enhance their chances of survival and reproduction. 
Sentence2: Animals learn to behave to increase chances of survival.

SOLUTION: behave

PROBLEM: Sentence1: Prokaryotes are single-celled organisms that lack a nucleus. 
Sentence2: Bacteria are single celled organisms that do not have a nucleus.

SOLUTION:
nucleus