What does one say or do if they think that a person's idea, behaviour, etc. is crazy to their mind?

For instance:<br>
In Germany, when someone dangerously passes you, you will show them by tipping your finger on your forehead(or temple) that they "*has a bird*" <sub>(literal translation of "einen Vogel haben")</sub>. This action is called "*jemandem den Vogel zeigen*", literally translated: "to show someone the bird".

I found the phrase "[bats in the belfry][1]" which has a similar meaning to the German phrase. I haven't heard it before, and I'm not sure if this phrase is in common use.

What do you usually say? Do you also have a gesture like tipping your finger on your forehead/temple? And is it insulting by law?

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I'm interested in both American and British phrases and gesture.

Just as side note: In Germany it is highly controversial if it is an insult by law or not. There were several court decisions pro and contra.


  [1]: http://www.phrases.org.uk/meanings/bats-in-the-belfry.html