In an effort to be easily accessible when you need her, Siri's default setting is to ignore your iPhone's passcode. As soon as you hold down the home button, you have full access to your mechanically-voiced personal assistant. She can help you find a restaurant, convert measurements or remind you to grab some milk on your way home.
But consider an alternative scenario, when a nefarious stranger gets access to your handset. By just holding down the home button, they can ask Siri to pull up your home address, display your girlfriend's phone number, send an email to your mom and even tell them if you have any appointments scheduled for the day—all while your phone is passcode protected.
Luckily, there's a simple fix to nix un-protected access to Siri. Inside your phone's "Settings" app, under "General">"Passcode Lock," switch the "Siri" option to "Off."
Now anyone who picks up your phone (including you) will need to punch in a code before Siri can pop in to help.