The_Complete_Chronicles_of_Narnia_(volumes_1-7,_in_order_of_publication)

"Well, it took time coming up. I suppose it'll take a little time going back."
Digory made rather a fuss about agreeing to this, but he had to in the end because Polly
absolutely refused to do any exploring in new worlds until she had made sure about
getting back to the old one. She was quite as brave as he about some dangers (wasps, for
instance) but she was not so interested in finding out things nobody had ever heard of
before; for Digory was the sort of person who wants to know everything, and when he
grew up he became the famous Professor Kirke who comes into other books.
After a good deal of arguing they agreed to put on their green rings ("Green for safety,"
said Digory, "so you can't help remembering which is which") and hold hands and jump.
But as soon as they seemed to be getting back to Uncle Andrew's study, or even to their
own world, Polly was to shout "Change" and they would slip off their greens and put on
their yellows. Digory wanted to be the one who shouted "Change" but Polly wouldn't
agree.
They put on the green rings, took hands, and once more shouted "One -Two - Three -
Go". This time it worked. It is very hard to tell you what it felt like, for everything
happened so quickly. At first there were bright lights moving about in a black sky;
Digory always thinks these were stars and even swears that he saw Jupiter quite close -
close enough to see its moon. But almost at once there were rows and rows of roofs and
chimney pots about them, and they could see St Paul's and knew they were looking at
London. But you could see through the walls of all the houses. Then they could see Uncle
Andrew, very vague and shadowy, but getting clearer and more solid-looking all the time,
just as if he were coming into focus. But before he became quite real Polly shouted
"Change", and they did change, and our world faded away like a dream, and the green
light above grew stronger and stronger, till their heads came out of the pool and they
scrambled ashore. And there was the wood all about them, as green and bright and still as
ever. The whole thing had taken less than a minute.
"There!" said Digory. "That's alright. Now for the adventure. Any pool will do. Come on.
Let's try that one."
"Stop!" said Polly- "Aren't we going to mark this pool?"
They stared at each other and turned quite white as they realized the dreadful thing that
Digory had just been going to do. For there were any number of pools in the wood, and
the pools were all alike and the trees were all alike, so that if they had once left behind
the pool that led to our own world without making some sort of landmark, the chances
would have been a hundred to one against their ever finding it again.
Digory's hand was shaking as he opened his penknife and cut out a long strip of turf on
the bank of the pool. The soil (which smelled nice) was of a rich reddish brown and
showed up well against the green. "It's a good thing one of us has some sense," said
Polly.