Every solid is bounded by faces. A face is a flat shape like a square, a rectangle, a triangle or a circle.
Count which faces you need for a solid and how many.
Example: A cube has 6 faces. They are all squares.
So you need 6 squares and no other face.
Example
Cube
Circle 0
Triangle 0
Square 6
Rectangle 0
Box
Circle 0
Triangle 0
Square 0
Rectangle 6
For parents
What does this exercise practice?
Recognize the flat faces of a solid and work out how many there are.
Common pitfalls
Curved surfaces like on a sphere, cylinder or cone are not flat shapes and do not count as a square or rectangle. Only count the flat sides.
How to practice at home
Take a box or a game die and lay a sheet of paper on each side. Count the faces together.
Related: Corners, edges and faces, Cube Net