Research on learning has shown the importance of the learner's possibilities to discern what differs as well as what is similar when meeting new phenomena. But how does this kind of understanding develop when young children try to understand their environment in natural settings? The results of Tolschinsky's research (2003) about young children's understanding of numbers and letters before being taught are considered in this study. Tolchinsky's results showed that children can separate cards that can be read from those which can not be read. In this study the point of departure is variation theory, and Tolchinsky’s results are seen through the perspective of what children do discern, what they discern simultaneously and what kind of variation they seem to need to discern. The aim in this study is to describe how children understand the concepts whole, halves and quarters. Three children aged four, five and six years old have been interviewed before, during and after a playful instruction about the topic. The results show how children understand the phenomenon and what implications this can have concerning their future learning. Due to one of the results, the children understand halves as halves as long as both halves are visible, but as soon as one half disappears the remaining becomes a whole as it is one piece and not two previously connecting to each other.