How Do Solar Panels Work?

They’re an increasingly common sight on the tops of buildings and residential properties: sometimes blindingly reflective panels that absorb sunlight and convert it into usable energy. Noted futurist Ray Kurzweil thinks we’ll all be using them by 2028. But how do solar panels work? Your basic understanding of solar panels—that they convert sunlight directly into electricity—is true. It’s called the photoelectric effect, and it was first discovered in the 1800s. Scientists learned that photovoltaic (PV) cells made of selenium were ideal for this purpose. While they later turned to the semi-conductor silicon, the idea remained the same: Up to a third of the energy found in the sun’s rays could, theoretically, be harnessed for electrical power under the most basic setup, and over 40 percent for the more expensive and complicated models. Silicon crystals by themselves aren’t necessarily great for this purpose, though. They need to be altered with boron and phosphorus and layered. The layer of boron-altered silicon has fewer electrons than it needs to form a bond, so an electron “hole” is created. The layer of phosphorus-altered silicon, meanwhile, has too many electrons. The space between is known as the P-N junction. As the sun’s photons enters the PV cells and starts dislodging electrons, the charge attempting to travel through the P-N junction gets diverted to an external circuit that can then carry the power to an inverter. You may not realize that the power generated with this method is a direct current, or DC, the same type found in batteries. To use the energy in a home or building, it needs to be converted to AC, or alternating current, the type found in outlets. And this is the role of the inverter. This kind of green energy has numerous benefits. Electricity doesn’t need to be transported by a utility company to the premises, and excess can be returned right back to the grid. If you’re thinking of installing a system, however, there are some caveats. For one, the upfront cost can be significant—around $10,000 to $13,475, depending on where you live, after tax credits. While you may be able to cut those costs with state rebate incentives or financing, it may take some time to recoup that investment. On the other hand, once installed, solar panels don’t appear to require much maintenance. Without moving parts, they’re not likely to suffer from mechanical issues, although you may need to replace the inverter at some point, which can get expensive. A site like can also use your specific location to calculate how much a solar panel system might save you. So how do solar panels work? It’s one part the photoelectric effect and one part economy. If Kurzweil is correct, it won’t be long before we’ll all be well-versed in both benefits. Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at

Why Does Hand Sanitizer Have an Expiration Date?

Hand sanitizer does expire. Here's why.
Hand sanitizer does expire. Here's why.
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The coronavirus pandemic has turned hand sanitizer from something that was once idly tossed into cars and drawers into a bit of a national obsession. Shortages persist, and people are trying to make their own, often to little avail. (DIY sanitizer may not be sterile or contain the proper concentration of ingredients.)

If you do manage to get your hands on a bottle of Purell or other name-brand sanitizer, you may notice it typically has an expiration date. Can it really go “bad” and be rendered less effective?

The short answer: yes. Hand sanitizer is typically made up of at least 60 percent alcohol, which is enough to provide germicidal benefit when applied to your hands. According to Insider, that crucial percentage of alcohol can be affected over time once it begins to evaporate after the bottle has been opened. As the volume is reduced, so is the effectiveness of the solution.

Though there’s no hard rule on how long it takes a bottle of sanitizer to lose alcohol content, manufacturers usually set the expiration date three years from the time of production. (Because the product is regulated by the Food and Drug Administration, it has to have an expiration date.)

Let's assume you’ve found a bottle of old and forgotten sanitizer in your house somewhere. It expired in 2018. Should you still use it? It’s not ideal, but if you have no other options, even a reduced amount of alcohol will still have some germ-fighting effectiveness. If it’s never been opened, you’re in better shape, as more of the alcohol will have remained.

Remember that sanitizer of any potency is best left to times when soap and water isn’t available. Consider it a bridge until you’re able to get your hands under a faucet. There’s no substitution for a good scrub.

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Why Are Sloths So Slow?

Sloths have little problem holding still for nature photographers.
Sloths have little problem holding still for nature photographers.
Geoview/iStock via Getty Images

When it comes to physical activity, few animals have as maligned a reputation as the sloth. The six sloth species, which call Brazil and Panama home, move with no urgency, having seemingly adapted to an existence that allows for a life lived in slow motion. But what makes sloths so sedate? And what horrible, poop-related price must they pay in order to maintain life in the slow lane?

According to HowStuffWorks, the sloth’s limited movements are primarily the result of their diet. Residing mainly in the canopy vines of Central and South American forests, sloths dine out on leaves, fruits, and buds. With virtually no fat or protein, sloths conserve energy by taking a leisurely approach to life. On average, a sloth will climb or travel roughly 125 feet per day. On land, it takes them roughly one minute to move just one foot.

A sloth’s digestive system matches their locomotion. After munching leaves using their lips—they have no incisors—it can take up to a month for their meals to be fully digested. And a sloth's metabolic rate is 40 to 45 percent slower than most mammals' to help compensate for their low caloric intake. With so little fuel to burn, a sloth makes the most of it.

Deliberate movement shouldn’t be confused for weakness, however. Sloths can hang from branches for hours, showing off some impressive stamina. And because they spend most of their time high up in trees, they have no need for rapid movement to evade predators.

There is, however, one major downside to the sloth's leisurely lifestyle. Owing to their meager diet, they typically only have to poop once per week. Like going in a public bathroom, this can be a stressful event, as it means going to the ground and risking detection by predators—which puts their lives on the line. Worse, that slow bowel motility means they’re trying to push out nearly one-third of their body weight in feces at a time. It's something to consider the next time you feel envious of their chill lifestyle.

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