Here's a fun little experiment that my four year old has really enjoyed. Starting with a carrot top and no greenery we have been watching progress as the green shoots appeared and grew. We talked about how it needed water and sunlight.
Time: 5 minutes plus a few weeks growing
Age: Under 5's
Bowl or plate
Cut the top off a carrot, you only need the top part and can eat the rest.
Place in a little water.
Check progress after a few days and a few tiny shoots will have appeared.
Add a little water every day if it's running low and see how the green top grows.
We also tried changing position of the carrot top and saw that the side that faced the sun grew faster than the other side.