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: an animal requires warmth for survival. 
Sentence2: Light and heat and warmth are basic necessities for survival.

SOLUTION: warmth

PROBLEM: Sentence1: Plants exhibit phototropism, or growing toward a light source. 
Sentence2: Phototropism means growing toward the energy of light.

SOLUTION: phototropism

PROBLEM: Sentence1: Cirrus clouds can indicate the arrival of a warm front. 
Sentence2: Cirrus clouds can indicate the arrival of rainy weather.

SOLUTION:
Cirrus