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: a stopwatch is used to measure time. 
Sentence2: Time can be measured with a clock or watch.
measure

Sentence1: If there is sun, there is sunlight, there is day. 
Sentence2: Energy enters ecosystems from the sun.
sun

Sentence1: fluoride decreases tooth decay. 
Sentence2: Water decreases tooth decay.

decay