Chipotle Mexican Grill (CMG -1.43%) is about to give away a lot of burritos. The market darling of fast casual is celebrating its 20th anniversary with a 20-day promotion that begins on Saturday at the new Adventurrito.com website.

The site will offer up a daily puzzle through each of the first 19 days of the celebration. Every day, 20 randomly selected participants will win a year of free weekly burritos. We're talking about 52 certificates for every winner that can be exchanged for the fast-growing chain's signature burritos. On the final day of the contest, the 20 best performers who completed all 19 puzzles will win 20 years of free weekly burritos.

In short, we're talking about as many as 19,760 burritos combined for the 380 winners of the daily sweepstakes and another 20,800 burritos for the grand prize victors.

Let's not assume that Chipotle will be taking a hit on the 40,560 burritos. Many of the individual Burrito Bucks certificates will go unclaimed. Folks coming in to claim their free eats may very well bring friends or tack on drinks and chips to their orders. Still, it's a smart promotional move, even if diehard fans may argue that Chipotle doesn't need the marketing muscle.

Chipotle's long yet fast-moving queues are legendary in the quick-service industry. Comps have remained positive even during the darkest recessionary stretches, with comparable restaurant sales rising 1% in its latest quarter. Sure, that wasn't enough to beat out inflation. It's also a far cry from the double-digit spikes that Chipotle was showing off for eight consecutive years leading up to its 2006 IPO. But it's still impressive to see a company that can routinely keep the register receipts growing.

Noodles & Co. (NDLS 1.27%) -- the blazing hot IPO that's been compared to Chipotle even though it's only half as big and growing half as quickly as Chipotle was when it began trading in 2006 -- has also promoted its ability to keep customers coming. Noodles & Co. has reported positive comps in 28 of the past 29 quarters.

McDonald's (MCD 0.28%), on the other hand, learned its lesson the hard way. The company that owned Chipotle before spinning it off at its IPO ended a 10-year streak of monthly positive comps late last year, just as the world's largest restaurant operator tried to wean customers off its Dollar Menu items. McDonald's has since gone back to hammering home its value message.

We don't know the nature of the puzzles that will come out of Chipotle in the next few weeks, but the 1,450-store chain is using all of the viral tools available. It's encouraging sharing via email, Twitter, and Facebook by awarding extra content entries to those who promote the celebration.

You can expect your Twitter and Facebook feeds to be flooded with Adventurrito links in a few days, and that will only help justify the "word of mouth" nature of this promotion, which will probably be far more valuable than the costs to roll out as many as 40,560 free burritos.

Chipotle knows what it's doing.