Challenge: To make this
Out of this
Enter the Turn'Up, an ingenious tool from France designed to carry out that very task. (Models TU01 & TU02 are shown here below.)
The gadget's main components are 1. a larger outer cylinder (right side of picture below), 2. a smaller inner cylinder (left side of picture) and 3. an oblong blade attached to the central plastic part of the Turn'Up (upper right of picture).
To learn how the Turn'Up gets the shape of a cup from a fruit or veggie (potato, cucumber, carrot, apple, turnip, melon or other fruit), read the description with pictures that follows.
To start, slice a potato (or other fruit) in chunks as thick as you want the height of your finished cup to be
(thickness of slice = height of finished up)
Next, line up the Turn'up with one of the slices, clutch the Turn'Up, push it all the way through the slice, then twist it back and forth to finish the cutting.
You've just cut through the potato slice with a duo of metal cylinders, the inner one ending a drop before the outer one. That means that only the outer cylinder cut through the slice the entire way. The inner one isn't supposed to—if it did, you'd be left without a base on your veggie cup.
The potato slice is now in three cylindrical parts. Your goal is to keep the middle one. First you have to discard the inner and outer ones. The latter is easy to ditch: it's already detached. Avoiding the Turn'up's circular blades (they're sharp), hold it and use your fingers to push the hollow outer veggie cylinder off.
Now press down on the Turn'up, just as you'd push a syringe's needle, to remove what remains. Two important points here. 1: Since you have to push from the top—another spot with a blade—make sure its safety cap is on. 2: Don't push down with a single aggressive motion. Do it with taps: you need to be gentle to keep from tearing the part you need intact.
Finished, right? Not so fast. Your veggie cup is still attached at the base to the central unwanted part. Here's where the gadget's oblong blade comes in. Remove the Turn'Up's safety cap and place your vegetable cylinder flat on the plastic base that juts out slightly past the blade. With the veggie held in the palm of your hand, press until you've reached the part of the tool that blocks the veggie from going further, and STOP.
You're thinking: Hey, wait a minute, haven't I just removed the veggie's base? No. The shape of the blade allows you to get the veggie's solid center cylinder severed, while leaving—joined to the veggie cup— enough of the base to keep inside what you'll later use as filling.
Push the vegetable off the blade the way you pushed it on: you don't want to cut off more than you have already.
Final step: Insert the point of a paring knife in the solid core and dig it out.
The Turn'Up comes apart with a simple push and twist. That makes it easy to clean. Plus, disassembled it gives you a couple of extra useful tools: pastry cutters (shown here below) with two different diameters.