This is a classic – but I’ve never seen it done with anything other than celery. I love this picture!
1) Take Celery or Napa Cabbage (largest you can find) and slice a stalk halfway up the middle.
2) Place two clear containers close together. Add a few drops of food coloring in two different colors to the jars.
3) Add water to the jars to fill to about 1.5 inches deep.
4) Wait about 8 hours and observe the difference!
Plants use little tubes to transport water from where they have it (the ground, usually) to where they need it (in their leaves, where it is “breathed out” through little pores called stomata.
Do It And How 
I tried that with my kids, it doesnt work, I think we need more food colouring drops to make it stronger? Or lettuce itself will not absorb from side of lettuce so we tried again with them from centre inside lettuce where they re still fresh inside.