A rat king, or “roi de rats” in French, is a collection of rats whose tails are intertwined and bound together. It typically occurs when young rats, sleeping together in close quarters, get their tails entangled, forming a knot. The tied tails are prone to being encrusted by dirt, or stuck together by substances such as sap, sebum, blood, food, faeces or urine. When the rats wake up and attempt to scurry away from each other, the knot then tightens, trapping them.