Can You Solve the Green-Eyed Logic Puzzle?

YouTube // TED-Ed
YouTube // TED-Ed

If you like logic puzzles, you'll love this. If you dislike logic puzzles, this may frustrate you. Either way, the answer is difficult to sort out, but it is a fascinating bit of inductive reasoning.

In the green-eyed logic puzzle, there is an island of perfectly logical people (or in some variants, dragons) who have green eyes—but they don't know that. They have been imprisoned on this island since birth.

On the island, green-eyed people are allowed to leave, but only under dangerous circumstances—they have to go alone, at night, to a guard booth, where the guard will examine eye color and either let the person go (green eyes) or throw them in the volcano (non-green eyes). The tricks are: people don't know their own eye color; they can never discuss or learn their own eye color (only inference is allowable); they can only leave at night; and they are given only a single hint when someone from the outside visits the island.

Given all these conditions, it's tempting to start thinking that the answer has something to do with genetics, or changing eye colors, or secretly sharing information—but there is really no "trick" here skirting around the no-communication rule. It's all about logic, and specifically the concept of common knowledge (not the kind politicians refer; rather, the kind logicians discuss).

Enjoy this tricky puzzle, and be glad you don't live on a murder-island:

For more, check out this TED-Ed lesson page or check out this io9 puzzle based on the same puzzle (but specifically based on green-eyed dragons, apparently introduced by a Harvard professor).

Amazon's Under-the-Radar Coupon Page Features Deals on Home Goods, Electronics, and Groceries

Stock Catalog, Flickr // CC BY 2.0
Stock Catalog, Flickr // CC BY 2.0

This article contains affiliate links to products selected by our editors. Mental Floss may receive a commission for purchases made through these links.

Now that Prime Day is over, and with Black Friday and Cyber Monday still a few weeks away, online deals may seem harder to come by. And while it can be a hassle to scour the internet for promo codes, buy-one-get-one deals, and flash sales, Amazon actually has an extensive coupon page you might not know about that features deals to look through every day.

As pointed out by People, the coupon page breaks deals down by categories, like electronics, home & kitchen, and groceries (the coupons even work with SNAP benefits). Since most of the deals revolve around the essentials, it's easy to stock up on items like Cottonelle toilet paper, Tide Pods, Cascade dishwasher detergent, and a 50 pack of surgical masks whenever you're running low.

But the low prices don't just stop at necessities. If you’re looking for the best deal on headphones, all you have to do is go to the electronics coupon page and it will bring up a deal on these COWIN E7 PRO noise-canceling headphones, which are now $80, thanks to a $10 coupon you could have missed.

Alternatively, if you are looking for deals on specific brands, you can search for their coupons from the page. So if you've had your eye on the Homall S-Racer gaming chair, you’ll find there's currently a coupon that saves you 5 percent, thanks to a simple search.

To discover all the deals you have been missing out on, head over to the Amazon Coupons page.

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