You have in your hands two ordinary plastic cups. You demonstrate this by pouring water from one cup to the other.
And back again.
Now you stuff a handkerchief into one of the cups.
Cover the cup with a second handkerchief.
Stretch a rubber band over the rim of the cup, fixing the handkerchief in place.
Now place your hand under the handkerchief and yank out the first handkerchief that was stuffed in the cup. How did it pass through the cup?
In actuality, you had earlier cut a small hole in the cup. When you poured water into the cup, you simply covered the hole with the pad of your thumb. When stuffing the handkerchief into the cup, push one corner out through the hole, allowing you later to pull out the whole handkerchief.