To replace electrolytes lost in vomiting or diarrhea:
1 qt water
1/2 tsp baking soda
1/2 tsp table salt
3 tbsp sugar
1 tsp salt substitute (lite salt) if available
Only for those over 12, under 12 you can use pediolyte, available at pharmacies without a prescription and I've even seen it at grocery stores.
It is vitally important to keep a proper balance of salts when you are rapidly losing them due to vomiting, extreme perspiration on a hot day, or other causes. And of course most people know but it bears repeating, that you shouldn't overdo water, even on a hot day, especially if you are not replacing mineral salts such as potassium, magnesium and sodium. In a few instances, people have died from drinking too much water as a result of a hazing incident or a radio contest. These tragedies were perfectly avoidable.