Well, yeah. But even if you don't know it was your ACL, you might be vaguely curious as to why you heard a loud "pop, felt a searing pain unlike any you've ever known, and experienced near-instantaneous, grapefruit-sized swelling. That, according to orthopedic surgeon Peter Laimins, is the most common sequence of events associated with complete ACL rippage. A subtler, partial tear might not be quite as cinematic, but you'll still know something's wrong. The "not know it till later syndrome occurs when some unwitting athlete figures a working knee couldn't actually be a busted knee. And while it's true that some lucky people don't experience immediate pain and swelling, there's another telltale sign: With a wrecked ACL, you won't be able to ski well. According to Laimins, "People can jog, run, or bike with an ACL tear, but they can't ski at a high level or excel at pivoting sports. So, if you twist your knee, feel no pain, but then suddenly ski like your grandma, it's probably time to see the doctor.