Acid and pH
Acid and pH is a scale that shows how sour or harsh a liquid is, from weak to strong.
Think of it this way
A row of jars sits on a kitchen counter, each holding a different liquid. The jars at one end hold vinegar and lemon juice, sharp and sour with many tiny sour particles packed inside. The jars at the other end hold soapy water, with almost no sour particles at all. The pH scale is a count of those particles in each jar.
Explaining acid and ph by grade level
Some drinks are sour, like lemon juice. That sour taste means the drink has acid in it. When you soak an egg in sour drinks, the shell gets weak and soft. Drinks that taste less sour do less harm to the shell.