This may or may not be too low-tech for you, but should be okay whatever your age group, provided you're a month or so from your science fair. As I recall, most science fairs are in February or March, but that's been awhile ago; my youngest kid is 21.
How about planting two identical seeds in two identical pots. Put one in a sunny place and the other in the closet, cabinet, or other dark place. Otherwise, treat them the same (same amounts of water, fertilizer or plant food.) Notate the difference and explain why there were different results with the two plants.
I am not sure what age group you are taliking about. I saw a fun one at my daughter's school. This girl had used coffee filters and a vaccum hose to prove there is little difference between high and low nicotine cigarettes. The girl wanted to get her dad to quit smoking. Just an idea. :)
When I was in high school, these were my science fair projects.
One year, I showed how different chemical solutions would burn different colors in a flame. For example, a copper solution makes the flame green whereas another one (which I have long forgotten now) would turn it red or purple.
The next year, I created a hologram, using a laser and a sandbox on innertubes. That was pretty neat! I'm sure I got the plans and the supplies from my teacher in both instances.
If you are stuck, I would talk to your science teacher and see what he/she suggests. It would also be helpful to know what sort of science you are interested in: physics, biology, chemistry, etc.
Ooo. I just had a fun idea. If you are into military tactics you could build an example of the Roman Phalanx and write up how affective it was in combat situations for the Roman empire. Not purely science but there is science in it. If you want to see how effective it was watch the movie 300.
Not knowing your age or your mechanical ability this could be challenging. You can go chemical and build a volcano and use chemicals to make it explode. You can go electronic and make a crystal radio.
If your a programmer you can write a program that takes a set of numbers and gives you the character set equivalent I.E. here is a random phone number 626-8147 it could spell.
m a m t 1 g p
n b n u 1 h r
o c b v 1 i s
hmmm... doesn't spell anything, bad example but still a good idea.
You could make and explain a water clock.
You could make and explain a fulcrum and give examples of how fulcrums are used today.
Give me more details about you and the science fair and I can probably come up with a lot more ideas. I time period would help too.
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