Van der Waals Forces
Van der Waals forces are weak pulls between tiny parts of matter that sit close to each other.
Think of it this way
Dry rice grains in a bowl do not stick together at first. But press two grains close, and a faint pull forms between them. This pull is too weak to lift them, but it holds them gently in place. That same faint pull acts between all small bits of matter when they sit close enough to touch.
Explaining van der waals forces by grade level
All tiny bits of stuff pull on each other just a little bit. This pull is very weak. It only works when things are very close. It is like how dust sticks to a wall. These small pulls help hold some things in place.