Can you solve it? An object that defies common sense

Today’s puzzle is about defenestration. Now there’s a word I’ve never used in this column before.
Design an object that cannot pass through an open window, but can pass through one that is almost half shut.
Here are illustrations of a cross-section of the window that make it clear what is required. The wall is blue, and the two sashes are orange.
The problem looks ridiculous – how can something fit through the smaller hole, but not the bigger one? It seems to defy common sense!
Yet there is an object – or possibly several – that fit the bill. Can you think of what it might look like?
And what about if the left sash was completely closed, making it flush with the wall – can any objects that cannot pass through the opened window now pass through?
I’ll be back at 5pm UK with the solution. Meanwhile NO SPOILERS. Instead discuss your favourite windows.
UPDATE: Read the answer here.
Source: Kvantik, a Russian maths magazine for schoolchildren.
In other hot news from the publishing world – my new book is out on Thursday! Think Twice emerged from a puzzle I set last year, which was the most popular puzzle in the history of this column. It set me on a quest to find more counterintuitive puzzles – that is, questions that seem blindingly simple but catch us off guard. I’ve collected the best ones in this book.
Think Twice: Solve the simple puzzles (almost) everyone gets wrong (Square Peg, ££12.99). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.
I’ve been setting a puzzle here on alternate Mondays since 2015. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.