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: Complex chemistry is needed to carry out all the functions of life. 
Sentence2: Chemistry is primarily concerned with matter, energy, and their interactions.
[A]: chemistry


[Q]: Sentence1: when a hemisphere is tilted away from the sun , that hemisphere receives less direct sunlight. 
Sentence2: Sunlight is delivered in small packets of energy called photons.
[A]: sunlight


[Q]: Sentence1: Examples are citric acid, tartaric acid, lactic acid and malic acid. 
Sentence2: citric compounds will turn the color of litmus paper from blue to red.
[A]:
citric