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: when a hurricane moves over land , that hurricane will decrease in strength. 
Sentence2: Hurricanes form over the warm tropical water in ocean and die down when they move over land.
land

Sentence1: Sound vibrations can be compared to pond ripples. 
Sentence2: dropping an object into water causes pond ripples.
pond

Sentence1: a black hole is formed by a high-mass star collapses. 
Sentence2: a star collapsing can be invisible.

collapses