So all you have to do is fill them up any way you please! I tried to make them all a little different... different colours, different sounds, different viscosities (look at me using a big word haha)!! etc. I was a little limited in choice because my bottles were so small... but whatever I found around the house that fit, went into one!
Yules loves to shake them and stare at the way the different liquids mix up or listen to the different sounds they make! She's had them for a while now and they are still a big hit! I highly recommend this DIY for your little one!