DISCLAIMER: I don't own it. I checked everywhere. Even under the sink! :)
I was thinking this is after Journey's End, the (10.5) Doctor and Rose, and about the piece of TARDIS coral the (10) Doctor gave them.
"Let's go stargazing," the Doctor said to Rose one night, and he whisked her out into the city.
Lights blocked out the stars, so they walked until they found grass. Collapsing on the ground, they laughed and stared at the bright stars above them.
"I wish we could visit them all," Rose whispered.
"One day we will."
He winked, she smiled. Minutes passed, and the stars seemed to be shining brighter.
"How many are there?" Rose asked quietly.
"To many to count."
"But if we'll see them all, how many will that be?"
The Doctor smiled.