Water xylophone

I've been slowly collecting identical glass bottles for this activity, and whilst you could do it with different shaped bottles and jars, it works better if you can use identical ones.  The idea is that bottles with different amounts of water inside vibrate at a different frequency (i.e. make a different tone/sound) when you tap them.  A note of caution which is probably obvious, but you need to closely supervise the use of multiple glass bottles and things to hit them with as there's a risk of broken glass.

Ready to start

I put out the bottles in a line, with three containers of coloured water (I used crayola bath dropz as they don't stain, but food colouring would be fine and you don't have to use colours, it just added a colour mixing element and a bit of fun!).  I gave my son big pipettes, and asked him to leave the first bottle empty and add a bit of blue to the next.

First attempt, with smaller water volumes

In the third bottle, he added the same amount of blue, and also added some yellow to make a larger volume of green.  In each bottle, he made a larger volume of liquid than the last, making either primary or secondary colours.  I got him to guess which colour he would make by combining pairs of primary colours, and he got some good fine motor skill practice with the pipettes as it was pretty repetitive.  You could pour to make it quicker, but a slow pace works well here at the moment as there's plenty of time at home to fill!

Greater water volume gave a bigger range of tones

When he'd got to the end, he wanted to try out hitting the bottles.  To my surprise he did it gently without being asked, and he noticed that the sounds were different, but there wasn't a huge range (about three musical tones) so we agreed to top up each bottle with some uncoloured water in a jug.  I helped him make sure there was more water in each bottle going from left to right, and when he tried the bottles again, the variation in pitch was easier to hear.  He correctly identified which was higher (least water) and lower (most water). 

Sounds are vibrations of the air, detected by our ear drums.  The faster the vibration, the higher pitch the note sounds.  The frequency of the vibration from a struck bottle depends on what is inside, and the speed that sound can travel through it - sound travels through water more slowly than through air.  So bottles with more water in produce slower, lower frequency sounds.

He enjoyed playing his new instrument and singing Christmas songs to his accompaniment (yes, it's February...!).