How to make picture "talk"?

If anybody has ever seen spongebob, the beginning shows a pirate with its mouth moving saying"Are you ready kids?".
Well, I was wondering if anybody knew how the creators of spongebob managed to make the picture look like it was talking by cutting out the mouth and replacing it with a video of somebody's mouth. Can anybody tell me how this is done?

Well you could technically just use a GIF editor heh but for real video you could use After Effects or Motion. Since you probably have neither i don't really know what you could use.

But its a basic use of key frames. Its quite easy. Just have the first key frame with the mouth normal and then a 2nd adjusted to what you want the mouth open to look like. Repeat them until you have enough mouth movement for the amount of text you want to be read.

well the opening of sponge bob is a little different.. i think they probably did that by covering the persons face with a chroma green or blue cloth with a cut out and he stuck his lips through it and they probably just keyed the lips over the pirate picture.. relatively simple effect.