How a Dishwasher Works
A dishwashing machine cleans dishes by spraying soapy water on dishes.
This is it. That is pretty much all it does.
A mind-numbingly simple concept, for sure. Not everyone seems to understand it, though, or at least they don't understand the implications of it, judging by the state of the dishes I sometimes find in the dishwasher after my roommate runs it. So, for my roommate, or for anyone else who wonders why their dishes are perpetually dirty, let's look at the significance of this revelation.
First, since we know what a dishwasher does (sprays water), consider what it does not do. The list is lengthy, for sure: it won't make coffee, it can't run a 4-minute mile, and it doggedly refuses to loan me any money. Only a few of the things that it will not do really concern us here. For instance, it will not scrub the dishes. It contains no sponges, brushes, rags, or SOS pads, and no arms or hands to wield them with even if it did have them. Again, it just sprays water. What this means is that it will generally not get rid of that caked-on cheese you let dry on your plate, and it will not completely get rid of that half-inch layer of peanut butter you left on the butter knife. (And why do you leave so much peanut butter, and sometimes just plain butter, on the knife? That's just wasteful.) So, the general rule is, if you would not be able to get most of the food off by spraying it with that hose at the sink, the dishwasher is not going to get it clean. The food will still be there when you open the dishwasher. Solution: scrape the food off before putting it in the dishwasher.
Another thing the dishwasher does not do (except in special circumstances, which we will cover in a minute) is move the dishes. The pots and pans will sit there in the same place and position you left them throughout the wash cycle. So, positioning does make a difference. If you jam all 20 spoons into one of the six small silverware holders, so that you have spoons sandwiched between spoons, the ones in the middle are not going to get cleaned. And for heaven's sake, don't stack cups like you do in the cabinet; how do you expect them to get clean when you have one cup completely covering another? The jets of water being sprayed have to be able to hit each item in the dishwasher. Also, since the water jets are mostly located in the bottom center of the dishwasher, and spray mostly up and out, you want all of the dishes to be facing either down or toward the center. You cannot leave cups or bowls facing up, because gravity still applies inside a dishwashing machine. Even if an upright cup does manage to get cleaned, the soapy water will not be able to drain down out of it during the rinse cycle. You'll be left with a cup full of dirty soap water, and a soap residue the next time you try to drink out of the cup.
The one caveat to the dishes-don't-move rule involves the lightweight plastic cups you like to use. You know, the logo-covered freebies you get at career fairs or as take-out at certain restaurants. The water jets inside the dishwasher are fairly strong, and they can toss those lightweight cups around like plastic bags in a windstorm. They will typically end up overturned and filled with dirty water, or laying in the bottom having been thoroughly cleaned on the outside but hardly touched on the beverage-holding inside. One solution: alternate the lighter cups with heavy ones, and place them close enough together that the heavy glasses can anchor the plastic ones in place. You should be putting the dishes in fairly close together anyway; your habit of skipping a space between each cup just cuts down on how much you can put in the dishwasher and requires you to run the thing twice as often.
To summarize: scrape off sticky or caked-on food before placing a dish in the dishwasher, arrange dishes so there is at least some hope that a jet of water will be able to reach the dirty areas, and make sure lightweight items are anchored down in some way (or just stop using cheap throwaway cups).
Next week, we'll discuss how to determine when your laundry is finished, so I don't have to always pull someone else's underwear out of the dryer when I need to run my own load 3 days later. Hint: it involves a buzzer.