There's a whole genre of web toy that only works because the other person doesn't know how it works — the rigged coin, the fake lie detector, the "mind reader" that always gets it right. I've built a small pile of these, and they're some of the most fun things on the site precisely because they're a little bit mischievous. The rule I hold myself to: harmless. These are for making a friend gasp and then laugh, not for actually deceiving anyone about anything that matters.
Here's the collection, sorted by the kind of trick.
Rigged games of chance
The oldest gag in the book: a game that looks random and isn't, with the outcome quietly in your control.
The rigged coin flip looks like an ordinary coin toss but has a hidden Always-Heads / Always-Tails / Fair switch — so you can call it right every time, or let your mark call it and be wrong every time. The rigged dice roller is the loaded-die version, where you pick the winning face in advance, and the rigged rock paper scissors always beats (or always loses to) whoever's playing. The rigged love calculator is the party-trick classic — type two names and get a "compatibility" percentage you secretly control, which is either the best or the worst thing to do at a sleepover depending on the names involved.
Fake detectors and mind readers
The second family pretends to measure something it can't — and sells it with a straight face.
The fake lie detector puts a thumb on the "sensor" and delivers a verdict you're driving. The ghost detector prank turns a phone into a very convincing haunted-house instrument. And there are three flavours of mind-reading: the mind reading prank (the "think of a number" routine that always lands), the crystal ball mind reader, and — the sneakiest — the way these guess what someone typed. They feel like magic because the method is hidden, which is the entire genre in one sentence.
Gotcha links
The last kind isn't a game you play with someone — it's a link you send someone.
The jumpscare prank lets you build an innocent-looking link (a "cute puppy video," a "check this out") that delivers a scream and a scare when they open it — you make it, you send it, you wait. The impossible button is the gentler one: a button that runs away, refuses to be clicked, and slowly drives the clicker up the wall. For the shareable ones, the victim links are built to expire after a day, and those pages stay out of search — a prank should be a moment, not a permanent thing with your friend's name attached.
Play nice
Every one of these is free, runs in your browser, and needs no signup. The only real instruction is the one at the top: keep it harmless. Gasp-then-laugh, not genuine deception. The whole roster lives in the prank tools hub — go get someone who'll get you back.