Sans blague! Tu blagues!
sah BLAAG! tew BLAAG! Click below to hear this.
No kidding! You’re kidding!
You have two ways to say either of these phrases: with excitement and a rising intonation, denoting admiration or amazement; or wryly and dryly, in a little more bass tone, denoting sarcastic agreement with a statement of obvious truth (Il fait chaud! –Sans blague! when it’s 90 degrees F. out). The two expressions are pretty well interchangeable.
Be sure you don’t make blague sound as if you are talking about a blog. Instead of dropping your jaw as if the doctor were poking a tongue depressor down your throat (say “ah”!), stretch the corners of your mouth sideways as if you were trying to fit a giant cookie into your mouth all at once. That will give you more of an /aa/ sound (“aaaugh!” as Charlie Brown would say). Try to hit somewhere between the two extremes, and you’ll be fine.
And if you really are joking around, tune in tomorrow. I’ll teach you a couple of jokes and riddles in easy French. Sans blague!