Take a straw and try putting it through a potato, without bending the straw. You will notice that the straw bends and fails to pierce the potato.
Now, follow the same procedure by covering one end of the straw with your thumb. You will notice that the straw will go right through the potato!
How does this work?
It is air that plays the trick here! Covering the straw at one end compresses the air inside the straw, building up force inside it. This makes the straw strong enough to pierce through a vegetable, as hard as a potato.