Dissolved Oxygen
Dissolved oxygen is the small amount of oxygen gas mixed into water that fish and other water animals breathe.
Think of it this way
Carbonated water has tiny gas bubbles mixed through the liquid. In a glass of sparkling water, gas bits float among the water. Fish water works the same way — oxygen mixes in and spreads as tiny, hidden bits. Fish pull that oxygen out the way you might strain small pieces from a drink.
Explaining dissolved oxygen by grade level
Fish need to breathe, just like you. They get air from the water around them. Cold water holds more air than warm water. That is why fish like cool streams.