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

Step 2: Building It

It takes aprox. 30 min to build it depending on how do you want it done ( breadboard , perfboard , no board , etc)
Follow the scheme included in this step to see how to make the device.
You can also use a 100K fixed value resistor for the circuit , but it won't work with all voltages.If you use a variable one , you can adjust it to work with the voltage that you're feeding the circuit with.

Step 3: Finishing It

After building it you may want to put it in a case,but remember , you want to hide this somewhere so people won't see it , this means that you can use any ugly case that you find around your house.I did this , and i used a VERY old box from an electronic door bell-ish thing, which actually had a working 10 ohm speaker in it , which makes a really loud sound which gives you headakes.
So, get a box and put everything together.(Notice my ugly box?)
As you can see in the photos , i used a 9V battery clip for the power source because i had 2 battery holders with the same connections .(1 was giving 3V and the other 6V)