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The Wendy's Frosty. It's not quite ice cream and not quite a milkshake, but it's completely unique to their menu. And now that Wendy's has brought back its Frosty Key Tag promotion to raise funds for the Dave Thomas Foundation for Adoption, it's time we all understand exactly what makes a Frosty… well, a Frosty.

Luckily, Reader's Digest is here to explain the finer points of chain's longtime dessert, right down to the specific temperature at which a Frosty should be enjoyed.

The Frosty is one of five original Wendy’s menu items

The co-founder of Wendy’s, Dave Thomas, opened the first location in 1969 with a five-item menu, which included hamburgers, chili, french fries, soft drinks, and the Frosty. Thomas had previously managed a few Kentucky Fried Chicken franchises, where he cut down the size of the menu, increasing sales — and he followed the same idea with Wendy’s, Mental Floss reports. All of these items are still available on the menu today, and in the mid-2000s, Wendy’s restaurants annually served about 300 million Frostys, according to a Wendy’s press release.

The Frosty was one of only five items on the original Wendy's menu.

The Frosty was one of only five items on the original Wendy's menu. (John Shearer/Getty Images for Wendy's)

Frostys need a temperature between 19 and 21 degrees Fahrenheit

Keeping the signature consistency of the Wendy’s Frosty — a cross between a milkshake and soft-serve ice cream — is essential. In fact, Thomas wanted to recreate the milkshakes he’d had growing up that were so thick that spoons were necessary, per Wendy’s. According to a press release from Wendy’s, the Frosty is kept between 19 and 21 degrees Fahrenheit to achieve this consistency.

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The Frosty ingredients aren’t a secret — but the exact formula is

The Frosty mix consists of milk, cream, sugar, and cocoa, among other ingredients. Even though Wendy’s lists these ingredients on their website, it is hard to find a good copycat recipe — although that doesn’t stop food bloggers and Frosty fans alike from trying. The Daily Meal reports that the current Frosty formula is the same as back in 1969 except for a decrease in butterfat, according to Denny Lynch, former senior vice president of communications at Wendy’s.

The chocolate Frosty isn’t 100% chocolate

Yes, Thomas intentionally made the original chocolate Frosty taste less chocolatey because he didn’t want to overwhelm customers with too much chocolate flavor if they paired a Frosty with a burger, per Wendy’s. Instead, the chocolate flavor is actually a combination of chocolate and vanilla. Thomas originally had one Frosty machine, so he combined the chocolate and vanilla mix himself.

For four more little-known facts about the Frosty, continue reading the original article at Reader's Digest.