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.

Example Input: Sentence1: Sand and mud are examples of sediments. 
Sentence2: layers of sand over the remains of organisms over time can create fossils.
Example Output: Sand

Example Input: Sentence1: propulsion is used for flying by a spacecraft. 
Sentence2: When Apollo 12 took off, the spacecraft was hit by lightning.
Example Output: spacecraft

Example Input: Sentence1: Producers use energy and inorganic molecules to make food. 
Sentence2: Ultimately, all energy on the earth comes from the sun.
Example Output:
energy