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: Most soil nutrients are cations. 
Sentence2: decomposition increases the amount of cations in the soil.

A: cations
****
Q: Sentence1: All chemical reactions require activation energy to get started. 
Sentence2: Catalysts lower the activation energy.

A: activation
****
Q: Sentence1: global warming is when worldwide temperature increases. 
Sentence2: Mass extinction occurs in the ice age and global warming periods.

A:
global
****