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 Q]: Sentence1: Any antigen that causes an allergy is called an allergen. 
Sentence2: molds are antigens.
[EX A]: antigen

[EX Q]: Sentence1: Ice is frozen water. 
Sentence2: ice expands.
[EX A]: Ice

[EX Q]: Sentence1: Steel is the most useful of metals. 
Sentence2: When steel rusts it becomes orange.
[EX A]:
Steel