TASK DEFINITION: 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.
PROBLEM: Sentence1: Earth's Atmosphere Earth's atmosphere is the layer of gasses that surrounds the earth. 
Sentence2: weather refers to the conditions in the layer of gasses that surrounds the earth.

SOLUTION: earth

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

SOLUTION: Ferns

PROBLEM: Sentence1: First, limestone is a strong rock. 
Sentence2: contact between limestone causes it to smooth.

SOLUTION:
limestone