It's a comic strip convention that's been around for about a century -- but until it's pointed out to you, you never see it. And that was the point of a sequence Mark Tatulli used in his comic strip Lio, published August 24, 2016
Z's in word balloons denote sleep in comic strips. Tatulli turns that idea around. Instead of the Z being generated when the character falls asleep, it's a physical object that actually causes the character to fall asleep. And note how Tatulli saves the payoff until the last panel.
When Lio retrieves the balloon, he's holding it face down so the reader can't tell what it is until the reveal in the last panel. You can tell we're seeing the back of the balloon by the way the smaller balloons overlap.
That attention to detail we don't notice consciously, but without it, the gag doesn't work. This is real comic strip artistry.