It might not happen so much for primary teachers, but I am humbled on about a weekly basis by students in my 5th grade math class who are smarter than I am.
Case in point, this pizza problem. Do whatever you need to do to enlarge that picture. The work you will see there is flat-out brilliant.
In this problem, a class has won the PTO's pizza party for bringing in the most Boxtops For Education™. Each student gets their own personal pizza and eats a different fraction of the pizza. They eat thirds, fourths, eighths, twelfths, and sixteenths. The challenge was to put the fractions in order from greatest to least to find out which student(s) ate the most pizza, and then find out which table group ate the most pizza.
The pair of students who made this poster demonstrate two different ways to create equivalent fractions with a common denominator of 48: the "Bring to 48" table at the top in the center of the page, and the longer version on the right side of the page. (I didn't teach them either of these methods. They came up with them on their own. Brilliant, right?)
On the left side of the poster, they show their work finding an equivalent fraction for each of the children in the problem. They add each column to find out which table group ate the most, and they put all of the fractions/students in order (below the "Bring to 48" table in the center of the page).
Differentiation is important. While these two were engaged in solving this problem and demonstrating their work on this poster, I was working with a group of students who still can't independently make equivalent fractions in order to add and subtract with an unlike denominator. Others in the class were working on solving the pizza party problem, but they never got to the demonstration stage, or else their demonstrations were not nearly as elegantly organized.