Let's say you are trying to make the audience guess what a particular item is made up, like a chooey black desert. And you say:
''You'll never guess what this is made up''
''You'll never guess what this is made up of''
Which one is correct?
The correct phrase is
You'll never guess what this is made up of
meaning you are referring to the constituents, expressing the relationship between the parts and the whole.
If you talk about a dessert (not desert), then you'd better say You'll never guess what it's made out of. We usually use made out of when we talk about something that has been changed or transformed from one thing into another.
Made up of can be used like this: