You're going up for the winning header and... thunk! Your skull meets the goalie's fist instead of the ball. So what happens next?
Shake an egg, then crack it. The yolk oozes round the edges. That's your brain when the delicate tissues get bruised as they hit skull's inner surface. A few million brain cells are maimed (don't worry, there are 20 billion of them) and you're left in a heap. Omelette, anyone?
A knock-out usually only lasts seconds. But your neurotransmitters all fire at once, erasing your memory of events right before the blow. The American Academy of Neurology says any loss of consciousness should keep you on the bench until you've been symptom-free for at least a week.
Dazed and confused
Easy questions can gauge the severity of your knockout. If you insist David Beckham's the prime minister or Amy Winehouse is a fox, get to hospital for a brain scan. Shallow breathing, puking and slurred speech are also signs you need medical supervision. Your brain is also busy fixing itself by absorbing huge doses of glucose.
Robotic movements, legs like John Cleese on a bouncy castle? No, it's not another goal from Peter Crouch. As your grey matter swings into overdrive to recover its higher functions, your more primitive motor activity functions run riot. Don't panic – such conduct should run its course inside 30 minutes.
Cry me a river
The post-concussive damage to your brain's distressed neuro-transmitters may leave you with short-term mood swings and loss of libido. Stick to dumb action movies while you recover, just in case Jennifer Garner's latest rom-com sends you into crying fits – and you are unable to satisfy your astonished lady friend.