Riddle

Here's a riddle that I though was pretty good:

It's present everywhere, but occupies no space.
We can measure it, but we can't see it, touch it,
get rid of it, or put it in a container.
Everyone knows what it is and uses it every day,
but no one has been able to define it.
We can spend it, save it, waste it, or kill it,
but we can't destroy it or even change it,
and there's never any more or less of it.
—Jespersen and Fitz-Randolph