Children are active and creative mathematical learners in the early years. This article reports part of a Teacher-led Innovation Fund (TLIF) project that investigated children's mathematical activity at an early childhood centre and the first year of school. Teacher-researchers were from each site, and mathematical opportunities were identified in children's play, based on their interests or from teacher provocations. Examples of children's explorations are reported that illustrate collaborative, creative, and curious mathematical activity, similar to practices of mathematicians. The roles of teachers were crucial in recognising opportunities and contributing questions to sustain and extend conversations.