"The Whistle that Refreshes"

Fill a Coke bottle half-full of water and blow over the top. Remember the pitch. Tap the bottle with a metal object (such as a table knife) and remember that pitch.

Add more water to the bottle and try again.

The pitch of the sound you hear when you blow over the top is higher than it was when the bottle was half full. The pitch you hear when you tap is lower.

Why is there a difference?