Miscibility

Immiscible fluids pose few problems. If you have a pint jug containing half a pint of sand, you can top it up with water. Although in real life you’d be able to get more than half a pint of water in (because sand is a fluid, but not a liquid), in practice you’re not going to upset many players if you keep it to exactly half a pint. The same applies to adding fluids to containers that already hold solids: You may as well compute volumes assuming that none of the solid objects float, because even if you account for partial displacement you’ll never get it sufficiently accurate to satisfy anyone pedantic enough to check anyway. Immiscible fluids are implementation-friendly. Miscible fluids are implementation-unfriendly.

If you have half a pint of alcohol and add half a pint of water to it, what you get is a pint ¹ of weaker alcohol. If you added water to poison, you’d get a larger amount of weaker poison. If you added poison to alcohol you’d get something that was half toxic and half intoxicating. Mixing fluids involves mixing properties of fluids. This wouldn’t be so bad if it weren’t for chemistry ².

By default, all solids are immiscible with liquids. However, some solids are affected by fluids in a major way. You might be able to package sugar in a paper container, but water would rapidly disintegrate it. The result of the exercise would be water and sludge, neither of which has the properties that a paper bag has. With fluids, you can add an acid to an alkali and get salt and water; neither the salt nor the water has any of the corrosive properties of acids or alkalis. Thus, mixing fluids can involve not only the creation and destruction of objects, but also the creation and destruction of properties of objects.

The keener designers among you might at this point be envisaging a nice chemistry system with a wide range of properties that fluids can have and a series of rules that govern how two or more properties combine to create other properties from the list. This could actually work quite well, and explorers would certainly thank you for it. Whether it’s worth the effort involved when you could be doing something else is another issue, of course, and the fact that the recipes for anything that can be brewed will be on player web sites within days might take some of the fun out of it. If you have the time, though, hey, go for it!

One final point about liquids is that viscosity can be a pain. Apples and honey are immiscible, but if you dunk an apple in honey and take it out you’ll end up with a lot of honey stuck to the apple. Whether or not you want to make the apple a leaky exocontainer for the honey is up to you, but either way be sure you cover the inevitable player experiments with it (“How come honey drips off kittens at the same rate that it drips off apples? Surely the fur would retain…).