I suspect a lot of people wonder what the difference between light and dark soy sauce is. Well, the dark version is thicker and slightly sweeter and the light one is, you guessed it, thinner and saltier. Dark soy seems to be sold as the default version (i.e., if soy sauce isn't labelled then it is likely to be dark). So if you are substituting dark for light then you may want to add a little sugar and salt to the recipe, and even add a teeny amount of cornflour. And if substituting light for dark, then remove salt. If you're not sure whether your soy sauce is dark or light, an easy test is just to tip the bottle upside down and see how much sauce adheres to the sides, as below.