As fast as an eagle, as slow as a slug. As tall as a tree, as small as a bug. Sometimes in front, sometimes behind. Never on top, and always aligned. What is it?

Gleam, glitter, glow,

They're moving very slow,

Sometimes on a smooth surface,

Often searching they\'re way down,

Between hunderds of obstacles,

They can be seen in every town,

Always moving and searching their way,

Again and again, everyday,

They merge and seperate with their species all the time,

And they can also be found, in this little rhyme.

What goes up the chimney down, but can't go down the chimney up?