What free and open mean here.
No subscription, no ads and nothing to buy. Every pack is a plain file anyone can read. Here is what's true today, and what isn't yet.
Shipped, limited, planned.
We only call something open once you can use it.
Every pack is free
No subscription, no paywall and nothing to buy in the app.
No account
Open the app and start. Your progress is kept on your device. See the privacy policy.
Pack files anyone can read
Each pack is one JSON file, served from the same address the app downloads it from, with a version number and a content hash in the catalog.
Load a pack yourself
The Packs tab can load any pack file from a link or pasted JSON. It's checked against the same schema as the packs in the catalog.
Sources
Pack files record a source for 52% of fact cards today, but the app doesn't show them on the card yet.
Build and share your own pack
A public pack repository, a documented format and a way to submit a pack.
FlashFeed builds every pack today, with AI assistance, and each one passes review before it goes live in the app. Packs are versioned, so a correction ships as a new version.
A pack based on a book is our own study notes on it. It isn't affiliated with or endorsed by the book's author or publisher.
Found a mistake? Tell us through the support page.
The repository the packs are built in is private for now. Opening it, documenting the pack format, and letting anyone build, fork and submit a pack are planned. We'll list each one above when you can actually do it.
Fun with Numbers, version 2.0.7.
This pack's entry in the live catalog the app reads, exactly as it was when this page was built on 25 September 2026.
{
"id": "world-numbers",
"name": "Fun with Numbers",
"shortName": "Fun w/ Numbers",
"icon": "🔢",
"version": "2.0.7",
"contentHash": "sha256:851533c65734f693",
"description": "A playful tour of magnitudes — building heights, mountain elevations, river lengths, populations, cosmic distances, big money, US trivia, and record-setting extremes. Guess first, then find out how close you land.",
"language": "en",
"url": "/packs/world-numbers.json",
"cardCount": 482,
"lessonCount": 9,
"tags": [
"mountains",
"usa",
"population",
"distance",
"height",
"structures",
"space",
"superlatives"
],
"creator": "Flash Feed",
"forkedFrom": null,
"status": "ga"
}