Exploiting the special synergy between science and mathematics can deepen children's understanding of scientific concepts and ideas and provide a range of contextualised opportunities for children to apply their mathematical knowledge and skills. With some creative thought and careful planning, science activities can be developed so that they require application of mathematics. This article explores how a science investigation focused upon grouping (classifying) seeds and observing plant growth can be utilised in the assessment of children's competency in applying place value (partitioning), addition and subtraction and using appropriate units of measurement in solving problems. Taken together the evidence can support judgements made about a child's mastery of key mathematical areas. The activities described in this article demonstrate that children's experience of learning science can be enriched when application of mathematics is built into lessons.