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: Cell death occurs by activation of cell death genes. 
Sentence2: Aging is associated with the activation of certain genes.
activation

Sentence1: Heat and temperature are the same. 
Sentence2: a thermometer is used to measure heat.
Heat

Sentence1: Earthquakes can cause a tsunami. 
Sentence2: Faulting of rock in the Earth's crust can cause a tsunami.

a