Why are Liberals the first to try to stifle free speech?

Why are Liberals the first to try to stifle free speech?
Why do liberals preach tolerance and freedom of speech, but then try to stifle anybody who does not agree with them? They often get very caustic with those who don't agree with them and they think that is OK. However, if you disagree with them, it is not politically correct. Personally, I think they are the ones who are politically incorrect more often than any other group.