Well, then just sit on the bottom at the shallow end of the pool so
that your head is about a foot under water, your lungs about 2 feet
under. Then take a standard, one foot long snorkeling snorkel in your
mouth upside down, so that you can point it to the surface.
Try to inhale.
This won't suck up your lungs, but give you a good idea of the
principle.
That's why snorkels are so short, duh.
While you are in the pool take a thin garden hose to the deep end and
see at what pressure it collapses. Don't try to inhale through it, you
would just exhale quicker and deeper than you wanted.