Hot Flow Anomalies (HFAs) are transient explosive events observed near the Earth’s bow shock. They are associated with the passage of thin interplanetary current sheets that track sufficiently slowly across the bow shock surface to interrupt the cocktail of kinetic processes that mediate the shock physics under collisionless conditions. Under steady conditions, the bow shock reflects a fraction of the incident solar wind. These reflected ions return to the shock location and mix with the incident ions to provide eventual thermalisation and entropy production. A HFA is believed to interfere with this process by chaneling those reflected ions upstream, robbing the shock of its primary dissipation mechanism and releasing that energy in the form of an expanding very hot, low density core with a strongly deflected bulk flow. I will discuss MMS observations of a short (25s) HFA observed by MMS. The combination of temporal detail and comprehensive particle distribution measurements reveal that this HFA is relatively young – the solar wind core is never fully dispersed. There is no sign of the expected backstreaming reflected ions, although there are very distinct clumps of low to medium energy protons. The HFA appears to be sustained by the pool of suprathermal ions accelerated by the post-HFA oblique shock. Surprisingly, although there is strong electron heating within the HFA, there is a previously unobserved cold core population of unknown origin.