You’ll probably remember from your math lessons at school that you can’t divide a number by zero, even if you didn’t understand why exactly. If you try to divide a number by zero on a electronic pocket calculator, it’ll pop up with a message saying “Error.” If you try this on a mechanical calculator, it looks like you’ve ripped an irreversible hole in the fabric of space-time.

This video by MultiGlizda shows the chaos that unfolds when you try to divide by zero on a Facit ESA-01 mechanical calculator with its casing off. Not only does it give an interesting insight into how these old calculators work, it also reveals the slippery nature of the number zero and its division.

As YouTube channel Numberphile explains, division is based on subtraction. If you want to divide a number by a second number, you simply subtract the second number from the first over and over again. For example, 20 divided by 5 would be: 20 - 5 = 15, 15 - 5 = 10, 10 - 5 = 5, 5 - 5 = 0. Since that took 4 subtractions, the answer is 4.

However, divide 20 by 0 and you’ll end up subtracting 0 from 20 an infinite amount of times. While that doesn’t mean that 0 equals infinity, it appears that the mechanical calculator is attempting to complete the infinite number of operations it believes it needs to complete the division. You can check out this Numberphile video if you want to know more about dividing by zero and why it doesn’t equal infinity.

Meanwhile, some say the Facit ESA-01 is still flicking through numbers to this day.