A very easy science demonstration that sparks interest in students is sprouting beans.
Take a piece of cardboard. Lay it on a plate. Add some water to the plate to be absorbed by the cardboard. On the cardboard lay 4 or 5 dried beans which can be navy beans, lima beans, black-eyed peas, or similar. Leave the plate with the wet cardboard and beans on the counter. It will take about 4 or 5 days depending on the weather for the beans to sprout. First you will see the beans swelling up as they absorb water. Then you will see a tiny root coming out of the bean. Later you will see a tiny stem. Place the plate in sunlight or at least daylight.
This experiment can be kept over a week or two or maybe longer for students to observe. Suggest that they try it at home, too. This will stimulate their personal interest in science. Children and older students are always excited to do science projects.
It's a lot of fun to observe the beans each day to see what kind of progress they are making.
An initial question that might be asked of students as you show them the first dried bean you are going to use:
Is this a living thing? They are likely to say no, since it hard, dry, and seems lifeless. Then your demonstration can show them that life dwells within each bean, each seed.
This can lead to further ideas, such as planting a garden for conservation and for economy and for the future.