Your body has chemicals in it that help keep everything working the right way. One of these chemicals is called acid, which is kind of like the sour taste in lemonade. Another chemical is called base, which is kind of like the opposite of acid, like the taste of baking soda water.
Sometimes, our bodies can have too much acid or too much base. When this happens, we call it an "imbalance". Just like when you're trying to balance on one foot and you lean too far to one side and start to fall over.
Having an acid-base imbalance in your body can make you feel sick, just like when you have a tummy ache. And if it's not fixed, it can make your body not work the right way.
Doctors can tell if you have an acid-base imbalance by doing tests on your blood, kind of like how they check if you had a good breakfast by taking your temperature and looking in your ears.
If they find that you do have an imbalance, they can give you medicine or make you drink special fluids to help bring your acid and base back into balance, like when you need someone to hold your hand while you walk so you don't fall over.