The aim of this study is to describe in what way preschool activities contribute to the development of children's knowledge of different forms of numbers; symbols, ordinal and cardinal. Video -recordings from three different groups of preschool children age d 3 to 5 years old have been analyzed based on variation theory (Marton, 2015) to describe the children’s discernment of numbers. The activities were planned and enacted by preschool teachers. The teachers’ intentions were to teach the children numerosity (Hannula - Sormunen et.al., 2016). The activities offered learning possibilities during the activities, e.g. to sort digits by magnitude, value or just to match symbols. The video -recordings were analyzed to capture what as- pects of the numbers the children were offered to discern, and what aspects were not offered to discern during the different activities. Simultaneous discernment of numbers in different forms are important for children’s future mathematical performance (Hannul- Sormunen et.al., 2015). The fi ndings unveil the teachers’ challenges to design preschool activities offering children simultaneously discernment of num- bers’ different forms. The activities mainly make the children discern numbers as symbols to match or as magnitude, while the cardinal form occurred only to a limited extent.