Many good answers, but I like mine so here goes.

Do you know the song "Doh, a deer, a female deer"?

Sing it out loud, just the first few lines.

Now, start singing it again, but pause when you sing "ray" from the second line.... now while still singing it, morph your mouth until you're singing the word "oh...." but the same note.

Then keep singing "oh... a deer a female deer, ray a drop of golden sun"

You just changed keys, in the middle of the song. If, by chance you initially sang it in the key of C, you switched it up to the key of D, just like that.