The first amendment states that "congress shall make no law abridging freedom of speech." the supreme court, however, has ruled that it is constitutional to restrict some types of speech, such as

One I would have thought was not protected by the 1st Amendment would have been hate literature. Wrong. Hate is protected. So is something like treason unless it incites violence.

The things that fall outside protection by the 1st Amendment are
Unsuitable pictures of children
Violence or the true threat of violence.
Obscenity (obviously it has not been enforced. Or it's been tried and failed).
defamation 

These and not rigid exceptions. There are tests to determine if something is or is not protected by the first amendment.

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