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: Cnidarians have a hydrostatic skeleton. 
Sentence2: Jellyfish have a hydrostatic skeleton.
hydrostatic

Sentence1: rocks interacting with wind over long periods of time causes weathering. 
Sentence2: Most sands are weathered fragments of quartz.

weathering