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.

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

Ex Output:
spacecraft


Ex Input:
Sentence1: boiling is when liquids are heated above their boiling point. 
Sentence2: If the pressure is reduced, the boiling point is lowered.

Ex Output:
boiling


Ex Input:
Sentence1: Ice crystals resist evaporation. 
Sentence2: Ice crystals can resist the sun's heat.

Ex Output:

Ice