The secret is in the page edges
Every third page is blank, every third is line art, and every third is colored, but a riffle only lands on one group because the other two are cut short of your thumb. Move your thumb up or down the stack and a different group catches. Stage magicians call this the Svengali principle, the same gimmick behind trick card decks, and it's self-working: the book does the sorting, so even a nervous eight-year-old performer can't flash the wrong pages.
How performers use it
A classic kids' party routine: show the book blank, have the audience draw in the air, riffle again to reveal line art, then ask everyone to throw imaginary colors at the book before the final colored reveal. The prop does the work; the patter sells it. You can watch the effect performed in about thirty seconds in this TikTok demonstration — notice you never see the performer's thumb move.
Where to get one
Magic and novelty shops sell the standard version for roughly ten to fifteen dollars, usually under names like "Magic Coloring Book" or "Fun Book." Licensed versions exist too. The gimmick is identical across all of them; you're paying for the artwork.
Make one yourself
Print about thirty sheets in three groups: blank paper, line art, and finished versions you've colored yourself. Stack them in repeating blank/line-art/colored order, then trim the edges into three stepped widths, each group about an eighth of an inch shorter than the last, and bind the stack along the spine. Our printable coloring pages supply the line art, and the online coloring tool can produce the colored set.