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.

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

A: Ferns
****
Q: Sentence1: Examples of gametes are ova and spermatozoa. 
Sentence2: fertilization of ova and spermatozoa produces a diploid zygote.

A: ova
****
Q: Sentence1: loud noises can cause animals to startle. 
Sentence2: Fireworks that only make a loud noise are called firecrackers.

A:
loud
****