It is, almost without doubt, one of the most infuriating things that can happen to you in games.
You're riding out at front, holding on to a well-earned lead as you drift this way and that in Mario Kart. And then it strikes. Maybe you'll have spotted it on the mini-map, or maybe it won't be until you hear the ominous swirling and insistent beating that you know what's about to happen. Maybe, if you're playing with friends, they'll shout out with glee what's coming your way. "Blue shell! Blue shell!"
The blue shell - or spiny shell, as it's also known - was first introduced to Nintendo's series in 1996's Mario Kart 64, and it's been one of the more, shall we say, controversial elements in the company's entire oeuvre ever since. A homing missile that targets the player in first place, it's the ultimate leveller whose wider significance might also provide the solution to society's wider ills.
Mario Kart's "The closer to 1st place you are, the less useful power-ups you get" system is an ideal model for how our economy should work— XanaX Damage (@1YUNGSPAM) June 10, 2013
The thing is, it's also really frustrating. So, when I recently got the chance to sit with Kosuke Yabuki - the director of Mario Kart 7 and Mario Kart 8, the two most recent instalments in the series - I had to ask about the item and some of the science behind its inclusion.
"Something I personally really consider is the human emotion element of the play experience," he told me. "So for example playing Mario Kart - if you have something that feels unfair or makes you feel frustrated or makes you angry... Everyone is different in that respect. What you will feel is unfair might be different to someone else. As far as possible we want to avoid those feelings of frustration.
"Because everyone is different, and it's an emotive thing, you can't pin it down with a set formula. Ultimately, in the longer term, although everyone might feel on one particular day that they're so frustrated that they're not going to play any more Mario Kart today - keeping the experience enjoyable enough so that you might feel like that today, but the next day, the next week, you'll still go back to Mario Kart and still enjoy it."
More importantly, though, would Nintendo ever consider ditching the item?
"We're always experimenting with what new elements to introduce or what elements can be removed," Yabuki said. "We have tried - or we are trying - to see what the game's like without the blue shell. When we've experimented without the blue shell, actually it feels like something's missing. Like there's something not quite enough in the game. So for now we've kept it in.
"You know," he continued, "sometimes life isn't fair. Sometimes in life you have something where you feel that's not right, and that's frustrating."
So there you have it. Confirmation that life isn't fair and neither is Mario Kart, two things you probably already knew.