What's forever?

As a child, I remember, one of my teachers explained eternity as the time that a single bird would need to carry away a whole mountain if it flies by every 100 years and picks up one single piece of sand at a time. My answer was disbelief, since one day, even though trillions of years away, the bird would have completed its task, and the mountain would have flown away together with it. So what’s forever? I think it was this story that made me never ever again believe that anything in this world could be forever. Nothing and nobody have since succeeded to convince me of the opposite. Whatever we see, whatever we do, whomever we meet, has an expiry date printed in small letters somewhere, even though sometimes it takes you some time to find it.