Use porridge oats, a variety of bowls and spoons to play and explore how many spoonfuls of porridge oats can fit into each bowl. Do they notice that the smaller the spoon the more spoonfuls? Use this play as an opportunity for lots of discussion. The picture below will give you the idea.
If you don`t have porridge oats at home or do not want to do messy play you could use three different sized cups/bowls and do the same activity with water as a bath time activity instead.