One of the hardest concepts about fractions for students to remember is the terminology of fractions. The most common question I get is "Which one is on top, the numerator or denominator?"
Why it's important to know the nomenclature of fractions isn't as critical as knowing what the numbers on the top (how many parts you have) and bottom (total same sized parts) represent. However, since it probably comes up on the test, students probably should learn it.
When the question is asked, my usual response is: "Denominator starts with the letter "d" and so does the word "down". The other one is the numerator since I can't think of a word for the top part that starts with "n".
So it was in a 4th grade class studying fractions that I asked: Does everyone know what the top and bottom parts of a fraction are called?
All (in sing/song voice): "denominator starts with "d" and so does the word "down". nominator starts with 'n' which looks like an an upside-down "u" that stands for "up".
Me: "Wow, that's pretty good! Where did you guys learn that from?"
Dude in front: "You taught us the first part last time you were here and our teacher liked it and came up with the second part".
I am totally going to steal that for the next time this topic comes up.
If you found this post searching for why they are called the denominator and numerator, Ask Dr. Math does a pretty good job of it.