The easiest way to imagine particles is as small balls. These particles are in constant movement.
The hotter they are, the faster they move.
This can explain freezing/melting, boiling/condensing and lots more.
It is called the Kinetic Theory of Matter. Kinetic is something to do with movement.
In solids, the particles are arranged in a regular pattern, touching each other.
They attract each other with a strong force (because they are so small and so close). This means that they cannot change places. Remember above, the particles of matter are always moving. Since they cannot change places in a solid, they simply vibrate.
A good way of thinking about this is to imagine the pupils in your class as representing the particles of a solid. When everyone is in their places, it is like a solid. Everyone moves a bit but they don't swap places.
Solids do not flow. The particles in a solid cannot change places so a solid will keep its shape (unless it is broken).
Solids cannot be compressed. Since the particles of a solid are already touching each other, they cannot be squashed any closer.
Solids can be quite strong because the particles are close which makes the forces holding each particle to its neighbour strong.
It is not possible to pass through solids without breaking them because the particles are so tightly packed and cannot move out of the way.
Remember, there are usually exceptions to most scientific rules, we all know examples of weak solids.
In liquids, the particles are still pretty close together but not necessarily touching each other. There is no pattern. We say that they arranged randomly. They are moving about in all directions, changing places all of the time.
The forces holding the particles together in a liquid are not as strong as those in a solid. Remember, the particles are moving all of the time. In a liquid, you can see that it is possible for them to change places.