Magnetic nanoparticles have been studied since the 1950s but today are being proposed for a range of new applications in biomedicine and technology. Before one can use nanoparticles inside the body or for a manufacturing process, it is critically important to have characterised them well. We must know whether they will agglomerate, whether they will respond to external fields, and how much heat they can generate. Despite this, many of the magnetic characterisation tools used today were developed over 50 years ago and need to be updated. In this talk, Dr Karen Livesey will detail several puzzles that magnetic nanoparticle experiments presented, and how she developed new characterisation tools to solve them.