• Starting January 16, California Pizza Kitchen guests receive prize-bearing thank-you cards for dine-in and carryout visits only.
• The sealed envelopes, which must be opened by a manager, might include anything from discounts on food to a $50,000 cash prize.

There’s a valuable secret in every thank-you card guests receive from California Pizza Kitchen (CPK) as part of a new three-month promotion that launched January 16.

But there’s a catch: Customers can’t open their sealed envelope themselves. They have to bring it back to CPK, where a restaurant manager will open it.

CPK has begun handing out sealed envelopes with thank-you cards to its dine-in and carryout customers at every participating location. Each envelope contains a secret prize that can be redeemed on a future visit. According to CPK, prizes include discounts off a meal, free pizza for a year, a trip for two to California, and cash prizes from $500 to $50,000. The prize is revealed only when a CPK manager opens the envelope.

Related: Jolly’s Pizza has a funny way of taking phone orders

The promotion is running until April 23.

As a bonus, guests who sign up to join CPK Rewards during the promotion will receive a $5 reward and a free small plate for registering.

Thank-you cards aren’t the only thing CPK fans can look forward to in January. Starting January 17th, the brand is also launching a new signature cocktail called Endless Sunshine that will feature happy hour pricing of $7 all day, every day. Endless Sunshine is a twist on a margarita made using Milagro Blanco Tequila, Monin passion fruit, Monin peach, fresh lime, and Fresca. The signature cocktail also comes in a spicy version, upon request.