Which clue words helped you know that "Letter to the Future" is written in the third-person?

Tommy and Walter tossed the baseball back and forth to each other like a drum beat, catching it with their worn leather mitts. The dirt field behind the newly constructed library was their favorite place to play catch and practice their batting skills. When they were finished, they would slump down in the shade of the mighty oak tree that stood proud between the library and the field and cool off.
"It sure would be swell if we could throw this ball so fast that it went straight into the future," Tommy said, as he pitched the ball directly into Walter's glove.
"Yeah, like into the next century," Walter replied. He paused a moment to ponder this. "I've got an idea, Tommy! There is a way to get this ball to the future." He quickly peeled off his glove and threw it toward Tommy. "Wait here. I'll be right back."
Tommy, not sure what Walter was thinking, picked up his friend's mitt and plopped down under the tree using the mitt as a pillow. Walter returned minutes later, carrying a small shovel and an old tin can.
"Don't tell me you're going to build a rocket with that old can, Walter, and shoot our baseball into the future," Tommy said, peering at his friend.
"Nope. We are going to put our baseball in it and bury it. Right here. Under this tree," Walter said matter-of-factly.
"Are you out of your mind? How can we play baseball without a baseball?" He pushed himself up from the ground and watched Walter dig the shovel into the earth next to him.
"I think my brother has one we can use," he said, still focused on digging. "There's a pen in the can. Write your name on the ball next to where I wrote mine. Whoever finds it will want to know we owned it."
"Whatever you say. But, I still think you've lost your mind," Tommy replied. "Shouldn't we at least leave a note somewhere to let someone from the future know we've buried our baseball out here? Our one and only baseball."
Walter wiped the beads of sweat from his forehead with the back of his hand. "How about we put a letter in that baseball book we read last week? Remember the one you knocked down from the top shelf in the library because you couldn't reach it?"
"Good idea! That book was so high up no one will find it for probably a hundred years," Tommy said. "I wonder if people will even play baseball in the next century."
"I hope so," Walter said.