At around 17:50 in Episode 10 of The Quintessential Quintuplets, Uesugi failed to lift a log, then Yotsuba said, 'are you actually a boy, Uesugi-san'.
But, wait! Let's take a closer look at how Uesugi was trying to lift the log.
According to law of the lever, you really shouldn't lift a log from one side. Uesugi did well in all subjects, including physics, so he couldn't be unaware of this. The correct way to lift a log should be like this:
So, he didn't intend to lift the log by himself from the start. He planned the failure to get Yotsuba to lift the log with him. Then he could probably get the chance to talk with Yotsuba.
How about Yotsuba? She helped the sport club a lot, so it's impossible that she didn't know the common sense how one should lift a log. Yet she still thought Uesugi was too week to lift a log.
Is this a clue that Yotsuba was too simple-minded to get the message from others? Or there is other reason to say, 'are you actually a boy'?