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: Herbs are medicines and aids. 
Sentence2: herbs are used for curing people when they are sick.

A: Herbs
****
Q: Sentence1: a sifter is used for separating mixtures. 
Sentence2: a sifter is used for some recipes.

A: sifter
****
Q: Sentence1: the number of rings in a tree trunk can be used to estimate the age of the tree. 
Sentence2: Age of young trees is estimated from both trunk circumference and tree height.

A:
age
****