Fat Separator is like a measuring cup, with a handle on one side, but with a long spout on the other.
The spout isn't at the top of the jug, though, as you might expect at first: instead, it comes out of an opening in the bottom of the jug.
To use, you pour drippings from your roasting pan into the jug, and let rest for a few minutes. The fat rises to the top, because it's lighter than the juices, and the juices stay at the bottom, where the opening for the spout is, and now it all starts to make sense. As you tip the jug to pour it, the juices from the bottom come out first, because they're right there where the hole and spout are.
Fat Separators are clear so that you can see the juices. They are usually made of heat-resistent plastic with side markings that allow you to gauge how much juice you have. There are, though, a few brands made of glass, spout and all. Some have a strainer over the opening as well to strain out bits.
Almost all are dishwasher safe.
An alternative method, if the roasting was done well in advance of the meal, is to put the drippings in the fridge for a few house. The fat will congeal on top. Spoon it off, and discard into compost bin.
You can also use a spoon to skim fat off, but this isn't as thorough, and takes more patience.
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