Can't comprehend the 4th dimension? Carl Sagan explains it with an apple in this classic clip.
Sagan was a master science communicator.

Carl Sagan and a sliced apple
The concept of the fourth dimension seems beyond human comprehension. As three-dimensional beings, we are unable to see beyond a physical object's height, width and depth. What else could there be?
Enter Carl Sagan, revered as one of the greatest science communicators of his time. He possessed a unique gift for demystifying complex scientific concepts, making them accessible and thrilling for the general public. In 1980, on Episode 10 of the groundbreaking PBS show “Cosmos,” Sagan embarked on a mission to explain the seemingly impossible fourth dimension.
What’s excellent about Sagan’s explanation is that he uses simple and relatable objects: an apple and a Tesseract, or a hypercube.
Sagan began by discussing how a two-dimensional being living in a flat world would perceive a three-dimensional object like an apple.
“Imagine we live in this ‘flatland’/2-D plane with no concept of ‘up’ or ‘down.’ Then along comes a 3-D object like an apple. We do not even notice it until it crosses our plane of existence — and even then, we have no idea what the apple is,” Sagan explains. “We see only a fragment as it passes through our plane. There is no way we can comprehend the 3-D quality/dimension of the apple, because it is more than we can understand. We only have the evidence of what has passed through our plane.”
Sagan then related this two-dimensional experience of the third dimension to how we might try to understand the fourth. To do so, he used the Tesseract, a four-dimensional cube, to demonstrate how difficult it is for us to perceive or visualize dimensions beyond our own three. At this point, Sagan is asking the viewer to expand their minds to understand the fourth dimension metaphorically.
Sagan’s demonstration of the fourth dimension isn’t just a wonderful explanation of a scientific idea that many of us find difficult to comprehend; it’s also a great example of how to teach complex ideas by combining clear explanations with thought-provoking visuals.
This article originally appeared last year.
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An Irish woman went to the doctor for a routine eye exam. She left with bright neon green eyes.
It's not easy seeing green.
Did she get superpowers?
Going to the eye doctor can be a hassle and a pain. It's not just the routine issues and inconveniences that come along when making a doctor appointment, but sometimes the various devices being used to check your eyes' health feel invasive and uncomfortable. But at least at the end of the appointment, most of us don't look like we're turning into The Incredible Hulk. That wasn't the case for one Irish woman.
Photographer Margerita B. Wargola was just going in for a routine eye exam at the hospital but ended up leaving with her eyes a shocking, bright neon green.
At the doctor's office, the nurse practitioner was prepping Wargola for a test with a machine that Wargola had experienced before. Before the test started, Wargola presumed the nurse had dropped some saline into her eyes, as they were feeling dry. After she blinked, everything went yellow.
Wargola and the nurse initially panicked. Neither knew what was going on as Wargola suddenly had yellow vision and radioactive-looking green eyes. After the initial shock, both realized the issue: the nurse forgot to ask Wargola to remove her contact lenses before putting contrast drops in her eyes for the exam. Wargola and the nurse quickly removed the lenses from her eyes and washed them thoroughly with saline. Fortunately, Wargola's eyes were unharmed. Unfortunately, her contacts were permanently stained and she didn't bring a spare pair.
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Since she has poor vision, Wargola was forced to drive herself home after the eye exam wearing the neon-green contact lenses that make her look like a member of the Green Lantern Corps. She couldn't help but laugh at her predicament and recorded a video explaining it all on social media. Since then, her video has sparked a couple Reddit threads and collected a bunch of comments on Instagram:
“But the REAL question is: do you now have X-Ray vision?”
“You can just say you're a superhero.”
“I would make a few stops on the way home just to freak some people out!”
“I would have lived it up! Grab a coffee, do grocery shopping, walk around a shopping center.”
“This one would pair well with that girl who ate something with turmeric with her invisalign on and walked around Paris smiling at people with seemingly BRIGHT YELLOW TEETH.”
“I would save those for fancy special occasions! WOW!”
“Every time I'd stop I'd turn slowly and stare at the person in the car next to me.”
“Keep them. Tell people what to do. They’ll do your bidding.”
In a follow-up Instagram video, Wargola showed her followers that she was safe at home with normal eyes, showing that the damaged contact lenses were so stained that they turned the saline solution in her contacts case into a bright Gatorade yellow. She wasn't mad at the nurse and, in fact, plans on keeping the lenses to wear on St. Patrick's Day or some other special occasion.
While no harm was done and a good laugh was had, it's still best for doctors, nurses, and patients alike to double-check and ask or tell if contact lenses are being worn before each eye test. If not, there might be more than ultra-green eyes to worry about.