What you learned in school
Biology and health education classes taught that hair and fingernails continued to grow for days or weeks after death, creating a spooky image of continued bodily activity. Students learned this as established biological fact, often accompanied by stories of corpses found with noticeably longer hair and nails. Textbooks presented this as evidence of residual cellular activity after death, and it was commonly cited as one of the mysterious aspects of human biology that occurred during the decomposition process.
What we know now
Biology and health classes taught that hair and fingernails continued to grow for days or weeks after death, creating a spooky image of the body's continued activity. Students learned this as biological fact. Hair and nails don't actually grow after death. The appearance of growth results from the skin shrinking and dehydrating after death, making existing hair and nails more prominent and visible. All cellular activity, including the cell division required for hair and nail growth, stops when the body dies. This is another example of a logical-seeming explanation for an observed phenomenon being incorrect.