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: a sifter is used for separating mixtures. 
Sentence2: a sifter is used for some recipes.
sifter

Sentence1: Passive immunity lasts only as long as the antibodies survive in body fluids. 
Sentence2: A pregnant woman will pass antibodies to the fetus.
antibodies

Sentence1: Viruses are so small that they can be seen only with an electron microscope. 
Sentence2: Tobacco Mosaic is so small that it can only be seen with an electron microscope.

electron