Buffer Capacity
Buffer capacity is how well water resists a change in how acid or base it is when something new gets added.
Think of it this way
A big pot of soup stays at the same saltiness even if you add a spoonful of water. The pot holds so much liquid that the small addition barely changes the taste. A tiny cup of soup, though, gets watery fast from that same spoonful. Buffer capacity works the same way — a large body of water resists change better than a small one.
Explaining buffer capacity by grade level
Some lake water stays the same even when you add something sour to it. Other lake water changes right away. The water that stays the same is stronger at fighting change. You can test water from different lakes to see which one holds steady.