Why can drinking way too much water at one time kill you?

It flushes all the electrolytes out of your system. It's called water intoxication.

http://en.wikipedia.org/wiki/Water_overdose

Drinking too much water, too quickly upsets the balance of electrolytes in the bloodstream. Most notably Chloride, Sodium, and Potassium. These electrolytes become diluted when too much water is consumed. This causes the heart to beat erratically and ineffectively. This disrupts the flow of blood to vital organs which can potentially lead to death.

I would add one more thing to these really good answers here. Many people think that by drinking a gallon or more of water, that it will help them beat a drug test. A: this is almost always ineffective because modern drug tests are very sensitive and B: it is potentially lethal for the reasons described.

Also, those people that said drinking too much water will cause drowning are full of baloney. That is virtually impossible in a healthy and conscious person.

What you're speaking of is water intoxication. It's a potentially fatal disturbance in brain functions that results when the normal balance of electrolytes in the body is pushed outside of safe limits by over-consumption of water. Normal, healthy individuals have little to worry about accidentally consuming too much water. Nearly all deaths related to water intoxication in normal individuals have resulted either from water drinking contests, in which individuals attempt to consume more than 10 liters (2.2 imp gal; 2.6 U.S. gal) of water over the course of just a few minutes, or long bouts of intensive exercise during which electrolytes are not properly replenished, yet massive amounts of fluid are still consumed.

There are many high-risk factors, including heat stress, endurance sports, low body mass and gastroenteritis (for infants and children), astrogenics, and medical/psychiatric.

During the most severe cases, there are 3 treatments: diuretics to increase urination, saline to restore sodium electrolyte levels, and vasopressin receptor antagonists - a drug to restore the hormones.

That should cover it, right? XD

Water intoxication (also known as 'hyper-hydration' or water poisoning) is a potentially fatal disturbance in brain functions that results when the normal balance of electrolytes in the body is pushed outside of safe limits by over-consumption of water.[1] Normal, healthy (both physically and nutritionally) individuals have little to worry about accidentally consuming too much water. Nearly all deaths related to water intoxication in normal individuals have resulted either from water drinking contests, in which individuals attempt to consume more than 10 liters (2.2 imp gal; 2.6 U.S. gal) of water over the course of just a few minutes, or long bouts of intensive exercise during which electrolytes are not properly replenished, yet massive amounts of fluid are still consumed.

It flushes out the system of all the good things that your body needs to operate. It's why gatorade has sugar and electrolytes.

You don't "drown" yourself. It doesn't go into your brain. It doesn't water out your blood. It's the electrolyte imbalance.

No, your system can't get rid of the water fast enough, so your brain ends up drowning.

She basically drowned her insides.

I remember hearing something like that and the next day, one of my teachers told me how it killed her.

Electrolyte imbalance caused her death

consuming a lot at once is like over hydrating yourself. basically water intoxication, too much of anything is bad.to me its like basically drowning your insides if that makes it sense

What happens when you fill an empty bottle with water through a funnel? It overflows the funnel. There is another hole in your neck that leads to your lungs and you will drown yourself...

...............SCARY.................