Collect clean recycled cups, bottles, and jugs of various sizes and shapes. Have your child use them in the bathtub for filling and pouring water.
Once your child has had time to explore freely, invite him to talk about what he's doing.
Which one holds the most water? Which one holds the least? How do you know? What happens when you pour water from a big jug into a small cup? How many of the smallest cups does it take to fill the biggest one? Do any of the bottles hold the same amount of water? Would you like any other size cups to try?
As in any discussion, follow your child's lead!