When you put a duvet cover in a washing machine without buttoning it up any other items of bedding (e.g. pillow cases) inevitably end up inside it. Instinctively, I would think that the opposite would happen (although I have no rigourous basis for believing this) due to the contents of the duvet cover being pushed out of it as it rotates. Why does this (to me at least) counter-intuitive behaviour occur? (Or are my findings not reproducible?)
From the Dutch National Science Quiz 2006 (my translation):
Question 14: You put a duvet cover together with smaller laundry in the washing machine. Why, at the end of the program, all smaller laundry has twisted itself in the duvet cover?
Due to the left-and-right cycling of the drum
Washing machines predominantly cycle one way. To loosen the laundry, the drum sometimes abruptly turns the other way. Due to this opposite movement, suddenly a few liters of water bumps very forcefully into the laundry, and therefore the opening of the duvet cover will come to lie completely open. Smaller laundry falls in one piece at a time. As soon as the machine goes into centrifuge mode, the smaller laundry pieces are being pushed in further. It is very difficult for laundry that is in the duvet cover to get out.
NB: The false answers were (my translation):
Because small laundry pieces are more sensitive to water vortices than large items
Due to the area (size) difference between the duvet cover and smaller laundry
I would like to add, from personal experience, that it is very rare that all smaller laundry ends up in the cover. Sometimes half of it will get in there, and sometimes just the odd sock. But perhaps that's just me and my machine.