This is by far our favorite science experiment these days at Our Beautifully Messy House. It's incredibly simple and quick to put together and L loves it! Whenever she has a friend over and there's a lull in the action, I usually pull this one out to keep the hooligans entertained.
First, put some drops of food coloring around the bottom of the container. Remember that the colors will eventually mix, so place the colors accordingly.
Next, pour some baking soda over the little, food-coloring puddles until the colors are completely hidden.
Now, pour some vinegar into a small cup. This is a great place to stop and talk a little bit about the science behind this fun and fizzy experiment. Even though L is only 2 years old, we still talk about how it's an acid-base reaction that causes the fizzing. She loves tasting the baking soda and talking about how it's salty versus the vinegar that smells and tastes sour. (Bottom picture is after tasting the salty baking soda.)
Now for the fun part! Using the medicine dropper, squirt vinegar onto the baking soda mounds and watch the concoction fizz while the colors appear and mix together. It should continue fizzing as long as baking soda is visible and your child continues to squirt vinegar into the container.
Not only is this experiment easy to put together, but clean-up is a cinch too! Just dump the mixture down the drain and toss the cup and container into the dishwasher. The vinegar-baking soda concoction will help clean your sink too...an added bonus! I hope your kids love this one as much as mine do! Enjoy!