Caffeine isn’t the kickstarting jitter-drug you think it is. If anything, it’s a sneaky imposter.

First, a little biology. As your neurons fire throughout the day, a neurochemical called adenosine builds up in your body. The nervous system uses special receptors to monitor your body’s adenosine levels. As the day wears on, more and more adenosine passes through those receptors—and it makes you sleepy. It’s one of the reasons you get tired at night.

Caffeine, however, is a stealthy impersonator. It’s the same size and shape as adenosine, and when you sip your morning joe, your adenosine receptors can’t tell the difference. Specifically, caffeine attaches to the A1 receptor. With caffeine docked at the receptor, a lot of your body’s adenosine molecules can’t enter. It creates a traffic jam of sorts. With all that adenosine blocked, the caffeine keeps you from getting tired.

But that’s not where coffee’s kick comes from. With the adenosine receptor clogged, neurotransmitters like dopamine and glutamate can get a head start. Your dopamine levels swell, giving you a mild jolt of energy. In a way, caffeine is like a bouncer. It blocks the door, keeping the tired molecules out while the more stimulating molecules party on.

But the party can last for only so long. Caffeine may give you that much-needed morning boost, but it can also make you crash—hard. It takes about four cups of coffee to block half of the brain’s A1 receptors. With that many receptors clogged, the adenosine mounting in your body has nowhere to go. So when the caffeine wears off, all that extra adenosine rushes through your receptors. It takes a long time for your body to process the huge flow of new metabolites. And guess what? It leaves you feeling even groggier than you felt before.

In the early 1900s, Forrest Mars, Sr., the son of Chicago candy maker and Snickers bar creator Franklin Clarence Mars, worked his way through Europe learning the ins and outs of the candy business. He worked for Nestle. He worked for Tobler. He started his own little factory in England. He sold some of his father’s brands. Most importantly, he found inspiration. According to confectionery lore, Mars was in Spain during the Spanish Civil War and noticed treats frequently placed in soldiers’ rations. They were chocolate pellets coated with a hard candy shell that kept them from melting (these might have been, or been inspired by, the “chocolate beans” made by Rowntrees of York, England since 1882).

Upon his return to the U.S. in 1940, Mars sought out another son of a famed candy man to put his own spin on the Spanish candies.

Bruce Murrie’s partnership in the new venture was essential to the candy’s success during World War II. His father was William Murrie, president of the Hershey Company, which meant Bruce and Mars had access to Hershey’s sugar and chocolate stores at a time when the ingredients were in short supply. It also guaranteed customers - Hershey had struck a deal with the Army in 1937 to provide chocolate for U.S. soldiers’ ration packs.

The partners Mars and Murrie dubbed their new candy with their initials, and M&M’s soon found their way around the world with U.S. servicemen (along with the 4-ounce, 600-calorie “Ration D” Hershey chocolate bar). The story didn’t end sweetly for Murrie, though. When chocolate rationing ended after the war, Mars bought out Murrie's 20% interest in the product and went on to become one of Hershey’s biggest competitors.
LEAVING THEIR MARK

Even with their partnership dissolved, Mars and Murrie’s initials stuck as the candy’s name and, in 1950, was even printed on it. Today, the Ms are applied to M&M's in a process that Mars Inc. describes as “akin to offset printing.” Blank M&M's sit on a special conveyor belt that has a dimple for each candy to sit in, and roll through a machine where vegetable dye is transferred from a press to a rubber etch roller that gently prints the M on each piece.

The printer can stamp some 2.5 million M&M's an hour. Some candies make it off the line M-less, but Mars doesn’t consider these rejects. Minor variations in the shapes of M&M's, especially the peanut ones, make uniform stamping difficult, and the machine is set up to let some blanks slip through rather than mark every one and break some candy shells in the process.