You cannot add tomatoes to clam chowder.

The Law

While not a literal criminal statute enforced by police, a legendary 1939 bill introduced in the Maine legislature sought to make adding tomatoes to clam chowder an offense against the state. The culinary purity law was a direct attack on Manhattan-style chowder, affirming that true New England chowder must only be made with a cream or milk base. Though the bill was largely a humorous PR stunt, natives still consider tomatoes in chowder a deep moral failing.