Amylase (which breaks down starch) is secreted in the mouth…

Amylase (which breaks down starch) is secreted in the mouth of humans. Amylase functions in the mouth but stops functioning once it hits the stomach environment. (a) What is the best explanation as to why amylase does not function in the stomach? (b) What is likely happening to the enzyme to prevent it from functioning? (I am looking for a special word.)