All you need for this activity is some milk (the higher the fat the better--we used 2% and it worked ok, but homo would be better!), a container to put the milk in, some food colouring, dish soap, and some Q-Tips. Poor a thin layer of milk into the bottom of your container and then put in a few drops of each colour food-colouring.
Now the fun part!! Dip your Q-Tip into the dish soap and then touch the Q-Tip in the center of one of the colour drops and watch the colour shoot away from the Q-Tip making cool swirling patterns in the milk. You can re-dip in the dish soap and keep touching the Q-Tip all over your container to watch the awesome patterns that emerge.
You can also swirl the Q-Tip around in the container and watch the colours run away as well! The boy-o was totally fascinated by this activity, and loved watching the colour spread out across the container. Once it started to get a little murky we added a few more drops of colour to brighten it back up a bit and started over again with making the colours run.
The reason that this activity works is the dish soap interacts with the fat molecules in the milk, which is why using a fuller fat milk makes for a better experience. The activity is another great one for some quiet toddler play or a rainy day.
To make the activity last longer simply dump out the coloured milk and put a fresh new layer in the container. It requires very little milk, so even repeating the activity a few times is still a very cost effective science exploration. (especially if your milk is getting to the end of its shelf life and you don't know how to use it up!)
We'll definitely be filing this one away for use another day and I hope you and your little enjoy it as much as we did!