You know that materials have properties. Some are hard, some are soft. Some are shiny, some are dull. You can use these properties to make groups.
Sam has a marble, a sponge, and a rock. The marble and the rock are hard. The sponge is soft. Sam makes two groups: hard and soft.
A glass cup and a paper cup can both hold water. But the glass cup is see-through. The paper cup is not. They share one thing. They differ in another.
To group things, pick one property first. Then check each item one by one. Ask: does it have that property?
Pick a property, then sort things by it, and say why.
Which group fits a plastic bag: waterproof or not waterproof?