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: Leeches are external parasites. 
Sentence2: Leeches live on the surface of their hosts.
Leeches

Sentence1: propulsion is used for flying by a spacecraft. 
Sentence2: When Apollo 12 took off, the spacecraft was hit by lightning.
spacecraft

Sentence1: Foam type insulations require an overlay board insulation. 
Sentence2: foam materials can be used to keep warm.

Foam