Thomas Edison invented the Christmas lights in 1882 (previously, people would adorn their trees with candles…seems to be quite the fire hazard!). No matter how carefully you repack your Christmas lights each year, they will get tangled - Physicists have proven that any cord longer than 2 metres, then knots are almost mathematically guaranteed (as well as a temper tantrum for whichever family member is tasked with undoing them...).