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: Metabolic rate is the rate at which food is burned for energy. 
Sentence2: thyroid hormones increase the rate of energy consumption.
energy

Sentence1: DNA is made up of nucleotides. 
Sentence2: Mutations can change the structure of DNA.
DNA

Sentence1: Mating is generally preceded by courtship. 
Sentence2: reproduction is generally preceded by courtship.

courtship