As the spring heats up, more and more of us will be slathering on sunscreen before heading outside to enjoy the weather. What SPF should we look for? And what does that number actually mean, anyway? Let’s answer some sunscreen-related questions.

To understand how sunscreen works, it’s important to first understand how ultraviolet light works.

UV light can be broken down into three regions. UV-A rays have the longest wavelength and are the nasty ones that penetrate deep into the skin (down to your dermis) to cause skin cancer and premature aging. UV-B rays have a shorter wavelength and only reach the outer layer of the skin, the epidermis, but they’re the major culprit behind sunburns. Then there’s UV-C, which we don’t really need to worry about; the ozone layer, water vapor, and other elements of the Earth’s atmosphere absorb these rays before they can get down to us.

With that brief overview in mind, how does sunscreen work?

On two fronts. Inorganic compounds like titanium dioxide and zinc oxide reflect or scatter the UV rays, while organic molecules like benzophenones absorb UV rays to keep them from reaching your skin.

All of this leads us to SPF, that magical number we hope is guarding us against the sun’s nastier side effects. A sunscreen’s Sun Protection Factor tells us how effective the product is at deflecting the UV-B rays that lead to sunburns. (There’s no good measure for quantifying how well a sunscreen handles UV-A rays, although “broad spectrum” sunscreens offer protection from both UV-A and UV-B rays.)

SPF is calculated based on how much longer it takes skin treated with a sunscreen to sunburn compared to skin that hasn’t been treated. Thus, a sunscreen with an SPF of 15 should theoretically allow a user to remain in the sun 15 times as long before getting toasty. Put in more scientific terms, SPF 15 sunscreen blocks around 93% of the UVB rays that would otherwise hit our skin, while SPF 30 blocks 97% of the burn-causing UVB.

While these calculations are consistent – Title 21, Part 352 of the Code of Federal Regulations describes a rigorous methodology for determining SPF involving an “accurately calibrated solar simulator” – the numbers themselves can be a bit misleading. The actual amount of UV light that’s reaching us when we’re outside varies depending on factors like the time of day, what altitude and latitude we’re at, cloud cover, and reflection of UV rays by the ground. (The World Health Organization says snow can reflect upwards of 80% of the UV radiation that hits it, while beach sand can send back 15% of rays.)

Thanks to these variations, it’s hard to pinpoint a specific time at which your SPF 15 or SPF 30 sunscreen will stop being effective. As a result, regardless of the SPF you’re using, the FDA and other researchers advocate reapplying sunscreen at least every two hours or so. Moreover, the FDA and WHO advocate coupling sunscreen with other forms of sun protection, like clothing and shade.

How much sunscreen should I put on? (Or: Why is there a picture of a sunscreen-filled shot glass above?)

Then there’s another problem: even those of us who scrupulously apply sunscreen probably aren’t using enough; many people are using as little as one quarter of the optimal amount of sunscreen. How much sunscreen should you be using? We’ll quote the FDA because we love the imagery they use: “An average-size adult or child needs at least one ounce of sunscreen, about the amount it takes to fill a shot glass—to evenly cover the body from head to toe.” Add that to your list of reasons to take a shot glass to the beach.