Mental models and affordances of can openers

Miranda Capra / Consumer Products / Human Factors

A few years ago I bought a new can opener that leaves a smooth edge on the lid of the can so that you don’t have to worry about cutting yourself on the lid. The first time I tried to use it, it took me 10 minutes and I completely mangled the lid of the can in the process. Why? Because the new can opener look very similar to my old one, but worked in a completely different fashion. Let me show you some example can openers to explain what happened.

I had always used a can opener like (A), where the body of the can opener is positioned along the side of the can, and you squeeze the handles together and turn the knob to open the can. My new can opener was a lot like (B). It still had two handles that you squeeze and a knob that you turn, but you place it on top of the can instead of on the side. I tried to use it in the same orientation as my old can opener, and it chewed up the edge without cutting anything. There were several traditional human factors design issues at play.

Mental models, or our internal understanding of how something works: I expected to hold the can opener to the side of the can because that’s what I’d done before. If I’d seen a commercial or photo of the product in use I might not have tried to use it the way I did, but without those tips I went on my personal knowledge of how can openers work. Can opener (C) is also a smooth-edge can opener, but has been designed so that you hold it to the side of the can, like my old one, which matches my mental model of how can openers work.

Cognitive affordances, or design clues that show us how to interact with a product: the new can opener looked very similar to my old one, and didn’t have enough visual clues to show me how to use it properly. Can opener (D) has a flat surface that looks like it should mate with the top of the can; if it were curved, then you’d put it on the side of the can.

Physical affordances, or design that physically allows you to do something: the new can opener was physically able to engage the can in the wrong orientation, but wasn’t able to actually cut. Can opener (D) has clear cognitive affordances that indicate you should lay it flat along the side or top of the can. But if you try to lay it along the side of a can sitting on a table, you quickly discover that it’s too tall, it just can’t be done, the can opener doesn’t physically afford placing it along the side of the can. You have to place it along the lid, forcing you into the correct position.

If you ever used 5.25″ floppy disks, you’ll remember that 3.5″ disks were a huge improvement because they had better physical affordances. Floppy disks are basically flat squares, so you can insert them 8 different ways. The smaller floppies were slightly rectangular, and had some holes and a diagonal cut corner that meant you could only insert them one way. The cognitive affordances were still poor, and it still took several tries to figure out the right orientation, but only one orientation would slide all the way in.

USB plugs are all designed so that they can only be inserted in the correct orientation, but they have varying degrees of cognitive affordances to give you hints about the correct orientation. I find the tiny ones so small that I can’t see which way to orient them, and the big flat ones look so similar from a distance that I can’t tell, but the square ones I always get right the first try.

For another example of physical and cognitive affordances, check out our blog about the Sidekick LX.

1. Smells like more potpourri — the Human Factors Blog

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   12.08.2009

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