TASK DEFINITION: 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.
PROBLEM: Sentence1: Apollo 13 spacecraft is launched. 
Sentence2: Propulsion was used for flying Apollo 13.

SOLUTION: 13

PROBLEM: Sentence1: a stopwatch is used to measure time. 
Sentence2: Time can be measured with a clock or watch.

SOLUTION: measure

PROBLEM: Sentence1: Most invertebrates have a nervous system. 
Sentence2: narcotic analgesics act on most invertebrates to relieve pain.

SOLUTION:
Most