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.

Sentence1: Hydrogen bonds cause water to expand when it freezes.
Sentence2: Hydrogen bonds Hydrogen bonds are primarily electrostatic in origin.
Hydrogen

Sentence1: a pulley is used for lifting objects.
Sentence2: Resistance exercises are weight-lifting exercises.
lifting

Sentence1: the color of leaves on deciduous trees usually changes in the autumn season.
Sentence2: A peachleaf willow changes colors in the autumn season.
autumn