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 Input:
Sentence1: Ferns are vascular plants. 
Sentence2: when a fern grows, its size of leaves may increase.

Ex Output:
Ferns


Ex Input:
Sentence1: Secondary accidents are a major problem under driving conditions of reduced visibility. 
Sentence2: Bad weather increases the risk of car accidents.

Ex Output:
Secondary


Ex Input:
Sentence1: Fertilization occurs when a sperm and an egg fuse to form a diploid zygote. 
Sentence2: Haploid gametes unite to form diploid zygote.

Ex Output:

diploid