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

A: Sand
****
Q: Sentence1: First, the bottle is calibrated with a graduated cylinder. 
Sentence2: calibration is performed with a cylinder.

A: calibrated
****
Q: Sentence1: Navigation is a key element of space travel. 
Sentence2: space exploration is not possible without navigation.

A:
space
****