Engineer vs Imagineer

Frank is my literal, unimaginative child.  My sister took him to a playground once and said " Let's pretend the sand is lava!"  Frankie replys "but it's not lava.  It's sand."
"I know," says Heidi "but let's pretend. "
"But it's sand." Frank retorts.
Ollie, on the other hand, is extremely imaginative.  He is always showing me his "animals", making his stuffed animals come to life, and telling me stories.  Did you know Darth Mal is part rhino?  That is why he has horns. 
Last night, I played build a story with the boys.  We start with a character and setting then add a few sentences about what is going on.  I started then tossed the ball to Frank for his turn.  Jack, our character, was on the moon.  Frank says "and he flew home."  He passes the ball to Ollie.  Ollie says " he likes being home and plays with his toys."  Ball to me:  Jack decides he wants to fly again so he builds a rocket.  Ball to Frank:  and flies to outerspace.  Ball to Ollie: And he flies to Mars!  Then he meets an alien!  The alien says to watch out for the captain alien cuz he's mean but all the other aliens are nice.....
As you can see Ollie caught on pretty quick.  The next story we did poor Frank kept ending the story with his turn.  After a few revivals he gave up and didn't I know what to say.  Ollie kept giving his suggestions.  Seeing this happen last night I know Frank will need to exercise his imagination a bit.  I can see both boys as adults.  Ollie will think up crazy things to build and Frank will be able to use concrete materials to build them all.  Thus, my engineer and imagineer.