Primary students (K-2): number recognition, shapes, patterns, addition
Intermediate students (3-5): looking for patterns, addition, shapes
This is simply a pattern search. The sum of the numbers on all six sides are the same as the sums on the 1/2 diagonals (lines from the center to the edge). I also included a color pattern for the younger students.
"Both diagonal lines are a pattern between single-digit numbers and double-digit numbers."
"All lines leading from the number '2' add up to 20."
"The hexagon makes sic triangles."
"Numbers 1 - 19."