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: a wildfire changes an ecosystem drastically. 
Sentence2: Forest fires change an ecosystem drastically.
changes

Sentence1: Protein Protein is needed for tissue maintenance and repair. 
Sentence2: protein repairs cells.
Protein

Sentence1: a cavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone. 
Sentence2: Carbonic acid dissolves limestone by separating the calcium and carbonate and creating a liquid.

carbonic