This is basically the easiest science experiment you can do with your kids and creates a real WOW moment for them (plus, RAINBOWS).
Rainbow volcanoes requires four things:
Food colouring (ideally the primary colours: red, yellow, blue)
Droppers or syringes (I clean and save them from the kids' medicine when we're done with it)
I grabbed a couple aluminum pie plates (but a baking sheet would work too - just something with edges to contain the mess as food colouring can stain) and three cups. I put a bit of vinegar in each cup and added a few drops of food colouring and mixed it up. I had three cups that matched the colours we were using but if you don't, then clear cups would work well too so they can see.
I put some baking soda in a bowl with a small spoon for each kid, handed them a syringe and the coloured vinegar, and let them start experimenting.
So, in case you missed this in elementary science, when you add vinegar to baking soda it starts bubbling and fizzing and is really cool. A pile of baking soda with a little hollow in it makes a great volcano effect when you pour vinegar into the hollow.
Another level of this activity is the colour mixing. By using the primary colours, you can show your children how mixing red and yellow make orange, and so on. Miss M (3 3/4 years old) LOVED this part even more than the fizzing part.
Mr M (21 months) had a little trouble manipulating the syringe but could still dab it into the vinegar and put drops on the baking soda which he thought was great fun. He also enjoy using the syringe and spoon to mix it all up together.
Meanwhile Miss M was mixing and stirring like a mad artistic scientist long after Mr M wandered away. She spent a good 30 minutes on this before her grandmother showed up for a visit - otherwise she may have gone on all morning.
It took less than five minutes to prepare and, being a baker, I had everything on hand for this. Clean-up was easy too - except for a couple dabs of food colouring on the counter. Might be worth the time to lay out papers or something underneath first, but it wore off after a couple days of cleaning it up. Will definitely be putting this in the regular rotation of activities!