Hand the child a small handheld mirror. Show them the underside of a chair.
Hold the mirror flat in your palm, raise it slowly under the chair. Look down into it. The mirror shows the chair's underside without you having to lie down.
Quiet, focused exploration. Kids will go around the house "mirror-checking" the underside of every table, fridge, and rug. Reverses the everyday view.
Walk while looking only at the floor in the mirror — you're "walking on the ceiling." Lay the mirror flat on the floor and look down into the "hole."