Melted fats or liquid oils make very effective thickeners in watery mediums. Oil and water won’t dissolve in each other, so when you combine them the result is a mutual disaffection that runs all the way down to the molecular level. Cooks can force the two to mix by a liberal application of the whip, which has the effect of breaking up large blobs of fat into many smaller ones. The thing is that no matter how small the blobs get they’re still many times larger than water molecules, and so do a very effective job at slowing down their flow.
This sort of mixture is called…say it with me now…an emulsion. The down side of emulsions is that they can be rather short lived as oil droplets tend to like to recombine and separate out of the mixture. To prevent this other bits of molecular flotsam and jetsam called emulsifiers are often added to the mix to keep them separated. Where do we find emulsions in the pastry kitchen? Why in everything from butter to cake batters…and very handy things they are too.