Here's the science behind the bonk. Glycogen is your body's fuel source during aerobic exercise. Your muscles store a finite amount of glycogen (about 1,400 calories worth). Your liver also stores glycogen (about 700 calories worth), but it's used to fuel your brain. So, let's say you ride for three hours and burn 700 calories an hour and you're replacing 200 calories an hour with a sports drink. It'll look something like this:

Muscle Glycogen = 1,400kcal
hour one = 700kcal (riding) minus 200kcal (drink)
Remaining balance = 900kcal
hour two = 700kcal (riding) minus 200kcal (drink)
Remaining balance = 400kcal
hour three = 700kcal (riding) minus 200kcal (drink)
Remaining balance = -100kcal (yes, negative)

Hmmm......you keep riding so those calories need to come from somewhere but you don't have any muscle glycogen left. So your body draws on your liver glycogen. Eventually, no more glycogen to the brain and you BONK! Yep, you get stupid, confused, and your judgement suffers -- just plain dangerous.

Folks will use the term bonk to describe lots of things when riding -- fatigue, dehydration, too much sun, whatever. But in the truest sense, a bonk is the lack of glycogen to feed your brain. I personally think everyone should do it at least once. There's no better lesson in proper nutrition than to bonk.