Madeleine's Roller Coaster Idea

Ever since we went on vacation this summer, Madeleine has been determined to build her own roller coaster for use at our own home.  Today she offered up some ideas she had brain-stormed for engineering such a roller coaster:

MADELEINE: Mommy!  I think I thought of how we can make a roller coaster for our backyard!  So, we can get a cardboard box, and put someone INSIDE it, and then push it down the hill so it rolls over and over on its way down and it will feel like going UPSIDE DOWN on the roller coaster.
ME: Uh...
MADELEINE: What?
ME: Not such a good idea.
MADELEINE: Why not?  Not safe?
ME: Definitely not safe.
MADELEINE: Well...we can roll it GENTLY.
ME: Uh...
MADELEINE: Still not safe?
ME: Still not safe.
MADELEINE: Why?  Cause the person could fall out?
ME: Among other things.
MADELEINE: I know!  We can DUCT TAPE the box closed!

Yes, that makes this whole idea safer.  What person *wouldn't* want to be duct taped inside a cardboard box and then hurled down a hill??

Madeleine, like the true engineer that she is, took the problems in her design to heart and re-thought the logistics.  Tonight she presented me with option #2.

MADELEINE: Oh!  Mommy!  I think I figured out how to make the roller coaster work!
ME: Okay well tell me in the morning.  It's time to do your bedtime reading.
MADELEINE: So.  Mommy.   We'd use a cardboard box, and then DUCT TAPE the person inside it, and-
ME: Okay, time to do your reading.  Tell me later.
MADELEINE: Okay.  Hold on.  And then, like Daddy and another person would CARRY the box while walking down the hill-
ME: Time to read, honey.
MADELEINE: Mmm-hmm.  Okay.  Hold on.  So Daddy and another person would be CARRYING the box, while SLOWLY turning it upside down while walking down the hill and-
ME:  Good-night.  Do your reading. (shutting the door on her mid-sentence)

See, I foolishly think that if I just don't respond to her brainstorm on building a roller coaster, she'll forget about it.  I can't wait to hear iteration #3 tomorrow or whenever she comes up with the newest design tweak.  No matter what that tweak is, IT IS NOT GOING TO MAKE THE IDEA OF DUCT-TAPING A PERSON INSIDE A BOX AND ROLLING THEM DOWN A HILL any less of a liability.