What does knob mean in the context of honey and faucet?

In the book What If? there is part that the author mentions about the viscosity of fluid:

It's more like pushing your hand through a bathtub full of water than a bathtub full of honey.

And this illustration:

What does knob mean here? Beside the regular meaning of a round gizmo or a door knob, two other meanings are a prominent round hill (AmE), and a penis (slang), neither of which I think fit here. Some results of googling:

• knob faucet: just the faucet handler
• knob honey: basically nothing, except a honey dipper with a knob
• knob fluid: fluid adjusting knob of a spray gun

If the knob here simply means the faucet knob, then I don't understand the whole illustration. What is so special about that kind of knob that he has to ask why the faucet has it?

Neither of them fits the context. So what is the knob?