A Game of Marbles

"Come play with me,Grandma," my grandson said one Saturday morning. The rest of the house was still folded up in sleep. But the two of us were wide awake. So we sneaked down to the living room and rummaged through the chest of toys in the corner. He found a box of marbles and a few metallic cars.
"You want a game of marbles?" he asked.
"Sure!"
"The idea of the game," he said slowly," is to get the most points."
"Okay," I said.
"You can have these marbles here." He set out four red marbles for me.
"I'll take these marbles here," he continued,setting out four blue marbles for himself.
There was a pause.
"Go on," I nudged him."Explain to me how we're supposed to play this game."
He looked around the room, desperately searching for the next step.
"Have you played this game before?" I asked.
"Oh yes! Many times."
"You don't seem too sure about it."
"Just want to make it interesting!"
"Why don't we roll the marbles down your marble run!" I said.
He considered my suggestion thoughtfully. "No thanks," he said eventually."It's too easy."
He spied a metal soldier and a plastic elephant in the back corner of the toy chest.
"Now," he said, his eyes wide with enthusiasm,"we can really start playing." He placed the soldier two feet from my position on the carpet.
"Grandma, if you can roll the marble and hit this soldier, you can get five points!"
I sprawled on the carpet. I steadied my hand and eye and I aimed. Wham! My marble hit the soldier right in the center with a "pluck."
Sebastian jumped up with joy! "That's the way, Grandma!'
"I haven't lost my touch," I said. "Let's see you repeat that!"
"Five points for you Grandma," he said, a twinkle in his eyes. "And ten points for me!"
"Now...wait a minute.... "
"You used one of my marbles--a blue one! So double the points for me !"
"That's not fair," I protested."Shouldn't we have set out the rules before the game?'
No," he said, laughing like a little penguin. "That's too easy."