Proteins are assembled from a set of 21 amino acids. Our bodies can make some of these amino acids, but there are nine essential ones that we can get only from our diet. For them to be useful to our body, we need to eat foods containing all nine at roughly the same time—it's no good to take in a protein missing, say, valine, this week and then move to a valine-rich protein lacking, say, lysine next week (though, I hasten to say, good nutrition doesn't mean eating them all in the same spoonful). In general, meat-, fish-, and dairy-derived proteins are complete—they have all nine of the essential amino acids. But many plant-derived proteins are missing one or another of them. This isn't a problem if you're a careful vegetarian, since eating, say, both corn and beans will supply your body with all the essential nine. And soy proteins, like meat proteins, are complete on their own, which is why soy products are such an attractive alternative to meat or fish.