Show same stuff on external screen as my screen When I'm lecturing I want the exact same stuff that shows on my MacBook Air's screen to show on the external screen. How do I do that?

For just slides the regular settings are fine, but when I'm coding/compiling/debugging live they are a hassle like no other.

(Ps. would like suggestions for better titles/tags in the comments. Or if your rep is high enough just go ahead and modify whatever you'd like.)
You did not specify the OS you are using but in general in the display preferences (System Preferences &rarr; Display) you should look for a mirroring option.

Screenshot for Mountain Lion: