“My app is fine with a file because users only save one at a time.”
Common mistakes
Would you have made these?
Each card is something a real learner said. Decide whether it’s right before you read why. Most of them sound right, which is exactly what makes them common, and every one is taken apart in a lesson.
Why use a database instead of just saving files? 4 mistakes
“When my program saves a file, the save either happens completely or not at all.”
“Computers are so fast that scanning a file for a record is never a problem.”
“Real software always uses a database; files are for amateurs.”
Why is the Fourier transform so important? 6 mistakes
“A sound has a single frequency, the pitch you hear, so a note on a violin is just one wave at one speed.”
“Fourier analysis only applies to waves that repeat forever, like a held note.”
“Turning a signal into its frequencies loses detail, like blurring a photo.”
“An equaliser or any filter changes a sound by adding new frequencies that weren't there.”
“I can measure exactly when a sound happened and exactly what frequency it has, at the same time.”
“The FFT is a fast approximation of the real Fourier transform, so it loses some accuracy.”
Why doesn't the Moon fall onto Earth? 7 mistakes
“The Moon is too far away for Earth's gravity to reach it, so it just floats there.”
“The Moon is 60 times further away, so Earth's pull is 60 times weaker.”
“If the Moon were really falling toward us, it would have crashed into Earth long ago.”
“The Moon stays up because gravity pulling it in is balanced by a force pushing it out, like a tug of war that ends in a draw.”
“Something has to keep pushing the Moon forward, or it would slow down and drop.”
“The Earth sits still in the middle and the Moon goes round it.”
“Things on Earth fall by gravity, but the Moon and planets are held up by some other, heavenly rule.”
Why does salt make ice water colder? 5 mistakes
“Ice melts by itself once it is warm enough, and melting takes no energy of its own.”
“A mix of ice and water is always at 0 degrees, whatever you add to it.”
“Salt makes the ice water cold because the salt is colder than the ice.”
“The more salt you add, the colder the ice water gets, with no limit.”
“Salt melts ice through a special reaction between salt and ice, unlike other things you might dissolve.”
Why does kinetic energy grow with speed squared? 4 mistakes
“How much energy a push gives an object depends on how hard you push, not how far.”
“A car going twice as fast has twice the kinetic energy.”
“If I'm going twice as fast, I need twice as far to stop.”
“Kinetic energy and momentum are the same: both measure mass times speed.”
Why does a stock have a bid, an ask and a price? 4 mistakes
“A stock has one price, and that is what you pay to buy it and what you get when you sell.”
“The price I see on screen is what I will pay when I press buy.”
“If I put in a limit order to buy at 101, I will pay 101 a share.”
“The gap between bid and ask is a hidden fee that my broker takes.”
Why do mirrors flip left and right but not up and down? 4 mistakes
“A mirror flips left and right, but not up and down.”
“The reflection is a picture on the surface of the mirror, so it is as far from you as the glass is.”
“Mirrors seem to flip left and right because our eyes are side by side, not one above the other.”
“A mirror shows you exactly what other people see when they look at you.”
What does a p-value actually tell you? 5 mistakes
“p = 0.03 means there's only a 3% chance the drug doesn't work, so it's 97% likely to be real.”
“p = 0.0001 is way below 0.05, so the effect must be huge.”
“p = 0.06, so the diet doesn't work. The study proved it.”
“Green jelly beans came out p = 0.04, so there's only a 5% chance it's a fluke, however many colours they tried.”
“p < 0.05 means a 5% error rate, so 95% of significant results are real.”
How does HTTPS work? 6 mistakes
“When I send something to a website it goes straight from my device to the site, so only the site can see it.”
“Encryption is safe because nobody knows how the scrambling works.”
“To use a secret key, two sides must have shared it beforehand, in person or by some safe route.”
“A certificate is just a file the site makes about itself, so it can't prove who the site is.”
“If the padlock is showing, the site is safe and honest.”
“With HTTPS, nobody on the way, my internet provider or the Wi-Fi owner included, can see anything about where I go.”
How does an AI model pick its next word? 6 mistakes
“It reads what I type letter by letter, so of course it can spell.”
“It finds the answer in its database, or searches the internet, and types it out.”
“Scores of 3, 1 and 0 add up to 4, so the top one gets 3 out of 4: 75%.”
“It always picks the single most likely next word, so it should give the same answer every time.”
“Turn the temperature up and the model thinks harder and knows more.”
“It already has the whole answer and just types it out slowly for effect.”
How can you tell whether a molecule is an acid or a base? 5 mistakes
“A substance is an acid or a base, and that label never changes.”
“Every hydrogen in a molecule can be given away as H+, so any molecule with hydrogen is an acid.”
“If a molecule has an OH in its formula, it is a base.”
“A strong acid is one with a lot of acid in it, and a weak acid is a diluted one.”
“A 0.1 mol per litre solution of any acid has a pH of 1.”
Horsepower vs torque: which one do you feel? 6 mistakes
“Torque and horsepower are two different measures of how strong an engine is, so one can be big while the other is small, whatever the revs.”
“A low gear gives the engine extra power, which is why you pull away so hard in first.”
“Horsepower is how hard the engine pushes.”
“Torque is what pushes you back in the seat; horsepower is just a number for the brochure.”
“The car with the higher horsepower figure is the quicker car.”
“Electric cars feel so quick because their motors are far more powerful than petrol engines.”