Bats do it, dolphins do it, whales do it: measure distances using sound.
Now your iPhone does it too!
Bat Sonar emits high frequeny sonic pulses (chirps) and listens for echoes, measuring how long it takes the chirps to bounce off of something and return to your iPhone's microphone.
Easy to use: turn the speaker volume to max., point your speaker towards an object and turn on the Sonar.
US and Metric units available.
Please note that objects must be away 2.5 feet / 70cm from the iPhone!