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If a light switch is functional, it's connected to a live wire. In new buildings with professional electrical installation, there's a pretty low chance a light switch would shock you. But when you've got aging wires and DIY electrical setups (shout-out to all the do-it-yourself landlords), that chance gets higher. The metal screws on a light switch cover are especially liable to give you a jolt if a live wire wanders near.

We use metal for electrical wires because it's an excellent conductor; that is, when you send electrons through it, it doesn't put up much resistance. Water — or, more specifically, the salts and other impurities in water — is also a great conductor. That means that if you're fresh out of the shower and standing in a puddle, a shock from a light switch can pass quickly through your body and into the puddle, forming a circuit that electrocutes you in the process.

As a result, many building codes have stringent requirements for outlets and switches placed anywhere near water. In the UK, for example, regulations forbid electricians from placing light switches within 0.6 meters (2 feet) of a shower or bathtub. In a small bathroom, that doesn't leave a lot of wall space for switches. The alternative, which many UK bathrooms use instead, is to use a ceiling pull-cord. That keeps wet hands a safe distance from live light fixtures.

US requirements aren't so strict, which is why you're more likely to find interior light switches in American bathrooms. Independent circuits designed to shut off when there's a change in current — like what might happen if electricity were to surge through your body — help keep you safe despite that.

The next time you use a bathroom with an exterior switch, you can feel secure knowing that it's just there for your safety — even if it might leave you peeing in the dark.