"Daddy?"
"Yes Jack?"
"Will the world last forever?"
...
"No. Not forever."
"Why not?"
"Nothing lasts forever.  But the world will last so long that it practically seems like forever."
...
"Like...100 years?"
"Longer than that.  Much longer than that."
"200 years?"
"No buddy, much longer than that.  5 billion years."
...
"Woah. (Long pause.)  Can I talk to Mommy about this?"