Particles - we've come across this word before when we did sound. What are they? Well, they're tiny tiny tiny tiny tiny tiny little parts of matter - a physical substance (that means something that you could touch).
What do they have to do with solids, liquids and gases, I hear you ask?
Well, particles make up all of them! But they behave very differently...
In a solid, they're all packed together very closely and linked very tightly - they don't move on their own.
In a liquid, the links between the particles are less strong and they can flow and move over each other, fitting a container.
In a gas, they just float about all over the place.
Have a look at this video to see how particles make up different states of matter (it's aimed at slightly older children, but is very good for what we're after).