Diffusion
Diffusion is the way tiny particles spread out from where there are many to where there are few.
Think of it this way
You drop a sugar cube into a still glass of water. At first, all the sugar sits at the bottom in one spot. Over time, the sugar spreads out until it fills the whole glass. This is diffusion — particles move from a crowded area to an empty one.
Explaining diffusion by grade level
Have you ever smelled cookies from far away? That smell spreads through the air all by itself. Tiny bits move from where there are lots of them to where there are not many. This is how smells reach your nose.