If you were told to think about swimming in the ocean, could you visualise it, would you actually be able to see yourself doing it? Or what about a puppy running around the garden?
Most people will be able to conjure incredibly detailed images in their heads, but there is actually a small percentage of people who can't think of anything. And it really blew this woman's mind:
This is due to a condition called aphantasia, which is basically a kind of mental blindness and an inability to visualise images of people's faces, places and things.
They can describe the thing or concept they're thinking, about but there's no mental pictures to go with it.
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According to Aphantasia.com: "Aphantasia is the inability to visualize. Otherwise known as image-free thinking.
"People with aphantasia don’t create any pictures of familiar objects, people, or places in their mind’s eye. Not for thoughts, memories, or images of the future.
"We lack this quasi-perceptual 'picture-it' system completely."
It's so rare that experts believe it only affects between one and five percent of the population.
As such, there are plenty of people out there who have never heard of it.
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In a video posted to TikTok, 23-year-old fitness instructor Courtney Hubbard explained that her boyfriend had just told her about the phenomenon and that she couldn't get her head around it.
She said: "He's told me that he's one of them people who can't visualise it in their own head, as in he can't imagine himself on a beach, he can't visualise an apple on the floor in front of him if he shuts his eyes.
"Can somebody confirm this, because it's blown my mind and I don't believe him."
Well, it turns out there were plenty of people out there in the same boat.
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Loads of people piled into the comments to share their disbelief at the strange phenomenon.
"Wait I thought that everyone had a voice in their head and could also visualize things," said one.
Another chimed in: "It must be so peaceful in some peoples heads hahaha."
"I can’t believe people cannot visualise. It’s actually mind blowing," put a third.
While another added: "I’m so confused because now I’m trying to work out if what I see is normal or not normal."
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But they weren't the only ones baffled by it all; even those with the condition said they didn't realise it wasn't normal.
One person said: "I'm now questioning myself after 29 years. I can't see things when I close my eyes, just think about things using my brain. Isn't this normal??"
Dominic Smithers