Speculation and facts are a strange combination to assess in science. This makes assessment a special activity needing judgement far removed from a tick-box mentality. The process of doing practical science and applying knowledge equally support the child in developing scientific rigour and understanding. Consequently, assessment in science is not just concerned with outcomes. Scientific understanding is about processes rooted in planning, teaching and learning. This does not make for an easy life. Instead the teacher identifies and reflects critically on evidence of scientific understanding and knowledge. This is a creative activity in itself, requiring practice and several examples to show how this might be done.