Maybe the doormat was a fine place to hide a key a few decades ago (but probably not). It's possible fake rocks were secure when the first 1,000 people tucked their keys inside and tossed them in the dirt. And the door frame is still an ideal place to stash a key if your burglar is 4 feet (1.2 meters) tall.
But if it's the 21st century, and if your would-be thief is post-pubescent, you're going to have to change it up. When you hide your house key in an obvious place, you may be reducing your inconvenience in the case that you lock yourself out or need a friend to pick up the mail when you're gone, but you're increasing the chance you'll find yourself in the much more inconvenient position of being robbed.
It's best not to have an extra key anywhere on your property. The safest place for a spare key is with a trustworthy neighbor or two.
If you must hide one, be creative. If your friends can guess your hiding place on the first try, pick a new one. Perhaps a hollow plastic lily pad for the moat.
If you're constantly locking yourself out, you might just chuck the key and invest in a keypad or fingerprint door lock. Thieves will have a harder time locating your spare when you're away.
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