What plants sprout and grow the fastest?

[Deleted]

DS wants to be in his school science fair in 24 days (wish we'd had more notice.  Oh well, next year).   He's only six  so I think a plant growing experiment will be the easiest thing that he can understand and make observations about on his own.   We'd like to plant seeds then water one group with water, another with milk, another with juice, and another with soda.  But do we have time to  grow anything that will be observable that quickly?  I know our pea plants for our garden sprouted in our basement within a week (just yesterday) but I would want something that will actually show good growth, not just sprout.  I'd hate to have him end up with nothing to demonstrate at the fair after working on it for three weeks.

 I read sunflowers grow quickly. What about the pea plants? Will those grow fast now that they've sprouted, because we have more of those seeds? Any other fast sprouting and growing seed suggestions?
TIA

[statlerwaldorf]

Spinach grows really fast. I started mine about a week ago and they are already about an inch tall.