Apart from one to one correspondence, another skills that is important for a child in his pre-number experience is similarity and relationship between two objects. Common everyday activities that involve sorting are the beginning concepts of children developing math skills. 

Naturally, when children explore their environment they notice how things are similar, and how they are different. They begin to sort them by characteristics that have meaning to them. These characteristics could be things such as color, size, shape, texture and sound. Children then begin matching objects that have the same characteristics. It is easier for children to begin matching pictures after they have had experience matching concrete objects. As children begin to master their matching skills, they will try more complex math activities.

For example, three different children will likely sort a pile of buttons of varying shapes, sizes, colors, and materials in three different ways. One child may put all the round buttons in one group and all the odd shaped buttons in a different group. A second child might put all the metal buttons in one group and all the plastic button in a different group. And a third child might sort the buttons according to color or size. The particular organizational system is not important. What is important is that each child accurately sorts according to his organization system and is able to explain his thought process.

When we sort and classify objects, you determine characteristics that the objects share, and characteristics they don't share. In other words, we identify how they are alike and how they are different. These characteristics are commonly called attributes.