This is an oldie but goodie! ...and it will be new to your preschooler.
I showed my preschooler how plants take water up their stems through little tubes and deliver it all the way to the tips of the leaves with this easy project.
I just took and old jar and let him fill it with water about 3/4 of the way, then he added "A LOT" of food coloring and a stalk of celery.
... we started to see a slight change in just a few minutes...
... and the next day they had completely changed color. My preschooler loved it and it remained a centerpiece on the kitchen table for the rest of the week, and sparked many conversations about the science of plants :o)