This is the lid to a food processor. Note the teeny hole in the center of the, er, plug piece. If you put an egg yolk and a squirt of mustard and some salt and pepper and a scant tablespoon, combined, of lemon juice and vinegar and maybe a smashed up garlic clove into the food processor, turn it on, and then pour oil – a nice light grapeseed oil, say – into the plug piece with the device whirring merrily away underneath, the teeny hole will stream the oil into your nascent mayonnaise at just the right rate to not overwhelm the emulsifying power of the egg yolk. And assuming you stop pouring in oil once the texture is to your liking, you’ll wind up with about 3/4 of a cup of mayonnaise, which is plenty for a household where one member doesn’t like mayonnaise and makes “yuck” faces at the very thought of it.
You could use this hole to drizzle oil into the mayonnaise you’re making, too, but this top for the food processor is more of a pain to clean, so why would you dirty it unnecessarily?