A bong is a filtration device/apparatus generally used for smoking cannabis.
A plastic coke bottle
A small piece of piping – 5/10cm (or some kind of tube)
A socket from a socket set that will fit in the end of the pipe
Make a hole for the pipe about 1/3 way up bottle and insert pipe so pipe is bending down into the bottle.
Put the socket in the end of the pipe outside the bottle
Make a smaller hole in the bottle above the first hole
Fill bottle with water so that pipe is in water, but water level does not reach the hole.