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.

[Q]: Sentence1: Apollo 13 spacecraft is launched. 
Sentence2: Propulsion was used for flying Apollo 13.
[A]: 13


[Q]: Sentence1: bacteria cause food to spoil. 
Sentence2: Food poisoning occurs when spoiled food is eaten.
[A]: spoil


[Q]: Sentence1: a valley is formed by a river flowing. 
Sentence2: And from the spring there flows a river.
[A]:
flowing