This is the Larsen philosophy with their Norwegian jigsaw puzzles. A quality puzzle have to be made of thick board. This to make the child able to get a good grip on the pieces, and of course to keep the pieces from breaking. The number of pieces must match the illustration and the age group that the puzzle is made for.
The puzzles must have a tray and permanent edge to make them easier for the children to play with, and easy to store. Children will rather have an easy puzzle that they can make many times, than having a difficult puzzle. If a puzzle is too difficult, the child will loose its patience. When making a puzzle many times, the children recognize the shapes of the pieces and where they belong. Therefore, it is important to give the puzzle pieces many different shapes. By learning how to recognize the shapes of the puzzle pieces the children will more easily learn to recognize other shapes, such as letters and words when they learn to read.