The purpose of this article is to study and analyse how a teacher implements an outdoor realistic problem situation for children aged 4–5 in a Swedish preschool. By an ‘outdoor realistic problem situation’, I mean a situation initiated by a teacher in which children come into contact with mathematical concepts and in which the outside environment is used to encourage learning. The analysis and interpretation of the data has been influenced by the didactical situation theory and didactic contract is a key concept. The term didactic contract is seen as a metaphor, consequently a broader definition of the didactic contract is used to illustrate how the implicit and the explicit rules of a didactic contract affect the communication of mathematics between a preschool teacher and a children’s group.