For example, the other day I was peeing and though, "Is there less splatter by aiming my pee above the water line or below the waterline?" If I was Richard Feynman, I'd probably actually do an experiment. Now that may not be allowed in a school fair, but it's an example of a trivial everyday thing that one can wonder about, and design an experiment about, yet maybe hasn't been established as fact (like plants like water).
You would not see any effect, not with just 3 plants.
It's part of the problem - any effect obvious enough to show up with the small resources of a child, is also pretty obvious.
I would do different: Have kids reproduce famous known experiments, and get rid of this useless obsession with hypothesis first. Just do a "hey, that's interesting" type experiments.
Also a stealth observation is the hidden assumption that a negative or null result means the experiment was a failure.
Personally I've got a gut feeling that given fertilizer runoff problems, the beans sprouts grown in pond water would grow faster unless the pond was unimaginably pristine. A bored kid could settle this interesting discussion via experiment... better than being stuck with "water, tide, or pepsi, which do plants crave??"
I want to say that the "science" of the science fair is a sick parody of the real process of science, except the sick parody is itself now the real process.
One of the few things from grad school that still can cause me to momentarily crinkle my eyebrows in annoyance 12 years later is when I was in my machine learning class, and we were told to go use some automated decision tree learning software to go learn something from some data set. I deliberately chose a dataset and question that should have produced a null result, precisely to validate that the technique in question did in fact produce a null result. Despite the fact the professor completely agreed that I correctly converted the data, correctly formulated a hypothesis, correctly tested the hypothesis, and correctly determined the null hypothesis could not be discarded... I took a 50% deduction on the assignment. A thing that was ultimately irrelevant since I still got the max final grade in the course... what furrows my brow isn't the actual bad grade, but what it says about the mentality of what science is, even at that high level.
If that still doesn't sound scandalous to you... and it doesn't sound scandalous to a lot of people at first... consider the outcome if I had not chosen a data set that I expected to be null. Suppose I chose a data set that I expected to be able to learn something from, and I was wrong. (This happens a lot in the real world with real data, after all!) What does this mean? It means that 50% of my grade would have been dependent on me massaging the data until I did get a result. Yow! Real science should not be encouraging people to mine data until they get results like that. And one need not spend very long looking at how real science is done to see this idea is quite pervasive in "real science".
Real science is not compatible with the idea that a non-null result is "success" and a null result is "failure". Success and failure have to do with whether the result, whichever of those things it may be, correlates with reality. Ask a scientist this question directly and there's at least a decent chance they'll mouth words about how important the reality correlation is, but look to their actions and it's all "result = success".
I'm gonna be brutally honest here... my plan is that when the kids hit the science fair age, I'm going to tell them to just do what they need to do to get by, and explain to them what real science is. The two things are in direct conflict, and I'll take real science any day.
"But at the three fairs I’ve attended over the last several years, the unknown rarely makes an appearance. At my son's fair last year, at least a handful of students did the popular "experiment" in which the "scientist" waters plants with three different liquids—one of which is typically soda or detergent—to determine which is best for plant growth.
"Were you surprised that water made the plant grow?" I asked one child after she presented her experiment.
"No," she said."
Aka "Science = boring and predictable and uncreative" Which is pretty much the opposite of reality, although a fairly accurate example of K12 science education...
Try harder kids? I put 10 seconds into it and got the idea of pond water, fresh chlorinated tap water, and collected rain water?
Then again, some decades ago I successfully won the local grade school with what boils down to a grid of detergents/soaps vs sources of dirt. (edited to highlite that dumb project ideas are not a recent invention of 2015, I mean think about it, a hypothesis that clothes detergent cleans dirt from cloth the best is lame, although what cleans kitchen grease was a good question, I think hand soap won?)
There is an interesting side dish of some fairs in some decades have allowed research projects or "inventions", which is pretty lame. Do science, do a book report, or do an art project, but don't confuse yourself into thinking the three activities are indistinguishable.