The shortest answer to this question is: Nothing good, unless you wish to be featured in the Guinness Book of records and/or the nightmares of small children. Just ask Lee Redmond, who grew her fingernails out to a record-shattering combined length of 28 feet, four inches. Admirable though her dedication was—it took her 30 whole years to achieve this length—the end result was, shall we say, a little disturbing. Tragically for Redmond, a car accident in 2009 (that thankfully left her otherwise uninjured) broke all of her nails, and she has wisely decided not to regrow them.
There’s nothing stopping her should she change her mind, of course: Nails are resilient things, made of the same tough protein called keratin that makes up the horns and hooves of other animals. With the living tissue of the nail (the part that grows) safely hidden under the skin of the finger, your nails are free to continue growing at a rate of roughly 3mm a month, although this fluctuates depending on age, weight, sex and, weirdly, how long your finger bones are (you might have noticed that your middle fingernail grows faster than your pinky, and they all grow faster than your toenails). They do not, contrary to popular belief, continue to grow after death: This is a myth caused by the fact the skin becomes dehydrated and “shrinks” after death, causing the nails (and hair) to look like they’re still growing.
As much of a pain as dealing with hand talons on a daily basis would be, it’s actually more problematic to grow out your toenails, as you’d soon be unable to walk without injuring yourself. As per this interview with Dr. James Christina of the American Podiatric Medical Association, after around six months, your toenails will start to curl in on themselves. Since, when you walk, you push off with the ends of your toes, every step would force your toenails upwards, eventually causing the nail not just to break but actually tear right out of your skin.
When all’s said and done, it really is worth trimming your nails regularly—although if you’re one of the 23 percent of men who trim their nails by biting them, you may want to take a moment to think about exactly what’s living under your nails.