Although it feels like meat grinders have been around since the beginning of time, they are a relatively new toy. The first one was built in the 19th century, by Karl Drais; the same guy who created the velocipede - a proto-bicycle.

Every time I have to use one of these, I am facing the meat grinder's dilemma. In plain English:

Which way does the knife go in?

Aaaaah! You've been there too, haven't you?

Follow up:

It doesn't matter if it is an old-school hand-powered unit, or a fancy electric one, they all share a design flaw - only one of the 2 ways to assemble it is right.

If the knife is plugged in the wrong way, the meat won't be minced properly, while the machine will continue to work as if everything is fine.

There are several ways to solve the problem:

• Empirical - install it one way, then turn it over if you see that it doesn't go well;
• Theoretical - try to figure out how the blades will rotate and how they will interact with the rest of the machine and the meat.

Last time I faced this dilemma, my enemy was a Bosch meat mincer, which had two deceptive solutions to this problem:

• The sticker - a large one the front of the machine. My mom decided to keep it there, fearing it won't peal off without leaving traces (if you're reading this on a laptop, take a look at your keyboard and you'll know what I mean).
• The shape of the metal - the knife has a hole in the form of a frustum (think of a pyramid with its top sliced off). The cylinder to which it is attached has a base of the same form - so they look like they should "click" together.

The sticker was useless, as it depicted many things, except a diagram of how to install the knife correctly. "Frustum on frustum" appeared to be the only logical solution.

Moments later, my dad showed up and mentioned the "obviously right way" to install it, which was the opposite of what I thought. He made a point by emphasizing "obviously". I politely reminded him that just 6 minutes ago he was asking me about which button to click in a program he was using; I answered his question without using the word "obvious", but I digress.

Having thought about it, I figured that there is a simple solution to this problem. Take your time to think about it before moving on. Really, don't read further until you think of a way to fix this.

• Feedback - make a buzz or turn on a light if the knife was positioned incorrectly. It is too complicated and it increases the cost of the unit, it also adds multiple points of failure. Once the signal is generated, the person using the machine will realize something isn't right, but they won't know what exactly is wrong and most importantly - they won't know how to fix it. The assumption that they read the manual before using the machine is a bad one.
• Information in the interface - replace the sticker with a helpful one that shows how the knife must be placed. This is much better than forcing people to keep the information in their heads, or read the manual every time prior to using the mincer.

These solutions are only partial, because they still make it possible to install the parts the wrong way.

A better solution is to add some asymmetry and eliminate the problem altogether. Add a little pimple to the base, and make a little hole in the knife - such that it "clicks" when installed correctly, and it remains tilted when set up the wrong way. By adding a constraint to the design, we make it obvious which way it should go.

There's no need to have a degree in contact mechanics and friction to figure out how that works. As an added bonus, the meat mincer designer won't go to bed wondering "how many people use my product the wrong way and are unaware of it?".

Bon appetit!