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: Propane can be either a liquid or a gas. 
Sentence2: heat can be added to propane to change it's properties through the boiling process.
[A]: Propane


[Q]: Sentence1: plant reproduction requires pollination. 
Sentence2: Plant reproduction requires insects and hummingbirds.
[A]: plant


[Q]: Sentence1: Amphibians begin their lives in the water. 
Sentence2: animals that begin their lives in water breathe with gills then move elsewhere breathe with lungs.
[A]:
begin