Can you solve it? Think like a physicist: curtains block 50% of solar energy, but what about two layers?

During a summer heatwave, many people keep curtains drawn all day to stay cool. But the physics behind it is more nuanced than expected. As a new puzzle explains, a single layer of curtains stops only 50% of solar energy. Here's how it works: Solar energy passes through the window and is absorbed by the curtain. The curtain heats up until it reaches a temperature where it radiates as much heat as it absorbs. Half of that radiated heat goes back towards the window, and the other half into the room. In other words, the curtain lets 50% of the Sun's energy into the room.

Now for the puzzle: "Imagine a room with roller blinds and curtains. The blinds line the windows, and the curtains cover the blinds. There are thus two layers of protection from the Sun. What percentage of the Sun's energy gets into the room?" At first glance, you might think the blinds pass on 50% to the curtains, and then the curtains radiate 50% of that into the room, giving 25%. Incorrect! Think like a physicist. Consider the heat flow and the equilibrium state of the whole system (sun, blinds, and curtain). No complicated equations needed.

The puzzle was created by Alan Williams-Key and published in The Guardian's fortnightly puzzle column. The author asks readers not to post spoilers but instead discuss blinds versus curtains. Since 2015, the columnist has been setting puzzles every other Monday and invites readers to suggest their own.