instruction:
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: creating paper requires cutting down trees. 
Sentence2: Trees begin as epiphytes on host trees.
answer:
trees


question:
Sentence1: Male dogs use pheromones in urine to mark their territory. 
Sentence2: Pheromones are hormones that are thought to stimulate, through smell, sexual reactions in animals.
answer:
pheromones


question:
Sentence1: irradiating food can be used to kill existing microorganisms in the food. 
Sentence2: Using radioactive gamma ray sources, existing microorganisms in food can be killed.
answer:

existing