MY SHADOW

by Robert Louis Stevenson

I have a little shadow that goes in and out with me,
And what can be the use of him is more than I can see.
He is very, very like me, from the heels up to the head;
And I see him jump before me, when I jump into my bed.

The funniest thing about him is the way he likes to grow—
Not at all like proper children, which is always very slow;
For he sometimes shoots up taller, like an india-rubber ball,
And he sometimes gets so little that there's none of him at all.

What literary element does the author of this poem use to create humor?