Guides

What is a HEIC file, and why will your PC not open it?

Half the size of a JPG, and a patent licence is why Windows refuses it.

How to get a PDF under an upload limit

Most PDFs cannot be compressed at all. Ten seconds tells you which kind you have.

Why WhatsApp ruins your photos, and how to send one properly

It re-compresses every picture you send. There is one way around it, and it is not the quality setting.

JPG or PNG: which should you use?

Photographs, JPG. Hard edges or transparency, PNG. The reason is worth two minutes.

How to get a photo under a KB limit

The quality slider is the wrong first move. Pixels are the lever that works.

Are QR codes safe? What is actually inside one

Nothing inside one can run. The risks are the destination, the sticker over the original, and who owns the code.

Why video files are so big, and what actually shrinks them

Bitrate times length is the file size. Everything else follows from that.

MP3, WAV or M4A: which audio format, and at what bitrate?

128 kbps is about a megabyte a minute. Everything else follows from that.

Percentages, and the four sums people actually get wrong

20% off then 10% off is not 30% off, and adding 20% then taking it back leaves you short.

How to take a passport photo at home that is not rejected

The rule is the height of your head, not the size of the photo — and the shadow behind you is geometry.

How to make a ringtone, and why the cutting is the easy part

Android takes an MP3 straight. An iPhone wants an M4R, which is an M4A with a different extension.

Compound interest, in both directions

Savings growing and a card balance running away are the same sum. Only the side you are standing on differs.

What actually happens when a password leaks

Why one company’s bad week becomes your problem at forty other sites — and the single habit that breaks the chain.

What you can change in a PDF without Acrobat

Moving pages and stamping them costs nothing. Changing the words is the one genuinely hard edit, and this is why.

Are electronic signatures legally binding?

Valid nearly everywhere — but a drawn signature proves the mark is there, not who put it there. What that changes, and when it is not enough.

Why background removal cuts off hair

Hair, glass and plants come out ragged for a reason that is not fixable by trying another site. What causes it, and what to photograph instead.

Why OCR gets text wrong

Why the mistakes land in invoice numbers rather than in sentences, what actually ruins a scan, and how to photograph a page properly.

What the numbers on a barcode mean

Three fields and a checksum. Why “690 means made in China” is wrong, and how the last digit is worked out from the others.

Why date calculators disagree with each other

One month after 31 January is 28 February, 2 March or 3 March depending on the rule. Why the calendar refuses to be arithmetic.

What fitness numbers actually tell you

Why the level a fitness formula gives you is usually wrong, why the change usually is not, and which of the two is worth watching.

What has to be on an invoice

The blocks nearly every country expects, the two that people get wrong, and why “net 30” gets paid later than a date.

Subtitles: a separate file, or burned into the video?

A file the player draws, or pixels in the picture. What each costs, and why automatic captions get names wrong and sentences right.

Why your final grade is not the average of your grades

The same four marks give 82.5 averaged and 74.0 weighted. How weighting works, and what you really need on the final.

Does decoy pricing actually work?

A real 1982 effect, a famous 84%-vs-32% demonstration, and a 2014 attempt to reproduce it on real products that worked about once in eight tries.

How to save a picture that is only on the clipboard

Why Ctrl+V does nothing in a folder after you copy a picture, what your computer can already save on its own, and the shortest way round it.