Fifth grade is a state of mind. And when you are in that state of mind, you can be fooled even though you know you are being fooled.
I used to teach kids about elephant seals. Sometimes I would flop down in the sand and say, in an elephant-sealy voice,

"Okay, I'm an elephant seal, ask me anything you want."

Sixth graders would make a noise of disgust ("pffshhhhh") and say "This is stupid".
But fifth graders would shout,

"No you're not!"
And I would say, "Oh yes I am!"
"No Way! If you're an elephant seal, how come you're wearing Dave's* hat?!"
"He loaned it to me."
"No! you're not REALLY an elephant seal!"
"Yup. Go ahead. Ask me anything about elephant seals."
"Wait! You're not really an elephant seal, are you?"

And there would arise a nice suspension of disbelief. Remember that in fifth grade, most kids are at the tail end of the Santa Claus years.

* "Dave" is not the real moniker of SmokeyBarnable. For complex reasons I pile pseudonym upon pseudonym.