Fake water drops

This is a nice prank to play on your angry friends , because it will really annoy them .
What does it do ? Well , it generates a sound similar to the sound that a water drop makes when it's dropped in a bucket or any other plastic object.
You can adjust the frequency of the drops by rotating a variable resistor , and it works with voltages between 6v and 3v .

Step 1: Parts

The way it works is very easy , it uses 2 switching transistors , 1 PNP and 1 NPN , a capacitor , a resistor and a speaker.
So , what you need is :
--1 General Purpose NPN Transistor (prefferably 2N3904)
--1 General Purpose PNP Transistor (prefferably 2N3906)
--1 10 Microfarad Electrolytic Capacitor
--1 Speaker
--1 Battery holder
--1 Variable resistor (100 K ohm)
--Some Wires
--Your favorite soldering tool