We did this as a fun observation experiment to demonstrate circulation in plants.
Robbie added powdered food dye to a container of water.
He stirred it until the powder was completely dissolved.
He placed the celery stalk in the colored water. He wanted to eat it instead.
The next morning, our celery leaves had turned pink.
Mommy "dissected" the celery so that Robbie could see the capillary "straws" had been stained when the plant "drank" the colored water.
This wasn't the most exciting experiment because there was no instant gratification, but he was happy to see the results and tell his Daddy all about them, nonetheless.