Preschooler Science Kit

We just did a fun science experiment with my 4 year old DS last month. Get a half a dozen white gerber daisies and some food coloring. Get 6 glasses, mix a different color with water in each, cut the stems, and stick a daisy in each glass. oh and best to leave one with no food coloring, just water as the control. It was fun to see the color of the daisies intensify with each passing day. And it was interesting to see that some colors were more readily absorbed than others.
Have fun.

A mini globe, baking soda and vinegar, cornstarch and food coloring makes sidewalk paint, 4cups of flour and 1 cup of baby oil makes moon dough, glow in the dark solar system stuff for his ceiling.... Can you tell my 4 year old is into science? Lol.

Pots with dirt and some seeds.

What a fun gift!
Usborne Books has some great "Science with Water", "Science with Magnets" books that might be fun to explore. I used the water one a lot for play extensions with my preschoolers and they had a ball.

Some larger magnets, even one big horseshoe one, is really useful and can pick up a lot of items around the house, and can magnetize certain items to use for experiments.

The other suggestions previously posted are great. My son grew a huge sunflower and wow, he is proud. Planting is something that fosters an interest in the dirt and earth. Wheat grass seeds will grow indoors if the weather is harsh outside. We've also been using expired alka seltzer tablets in old film cannisters and making a mini-rocket with it.

Peg M from Canby and her husband have TOPS, a children's/school science kits website. It's worth checking into...

http://www.topscience.org/info/reviews.html

You can cut up pipe cleaners and put them in a thin container (pop bottle would work well...) along with a few other random things for him to 'find' that are non-magnetic. Then he can use a magnet to move the pipe cleaners around and find the treasures in the bottle. :)

Magnets in general are tons of fun! If you give him stronger ones, MAKE SURE to supervise him when using them... swallowing magnets can be extremely dangerous.

hot glue shut a lid on a water bottle with colored water and oil in it. or you can hot glue two water bottles together with colored water, glitter, beads etc in it. then show him hot to swirl it around to make a tornado :)

Science is everywhere.
Have him work in the yard (or patio) with you, planting seeds, watering, watching the weather, watching the plants grow, and talk about it.
Same thing in the kitchen, measuring, stirring, baking, talking about ingredients, why do we need yeast, salt, eggs, etc?
Taking care of animals, or babies (if you have a younger) also gets these lessons and conversations flowing.
Going on a nature walk, talking about the flowers, leaves, trees, caterpillars/tadpoles/ducklings, collecting rocks, ask him what he notices. Again, talk about it.
I love that age, they soak it all up, like curious little sponges. Have fun :)

You can get a relatively inexpensive microscope and telescope at most toy stores, and he's not too young to learn how to use them. A prism is also a great toy. XUMP.com is a great place to find science toys online, and you can search by age. (If you're brave enough, get him an ant farm!)

For home-made items, you can't beat water bottle rockets and water scopes, especially in the summer. To make a water scope, all you have to do is cut off both ends of a large coffe can or large plastic tube and cover one end really tightly with plastic wrap (use a large rubber band, like the kind that comes around veges to secure the wrap). This is great for exploring creeks and ponds -- instant underwater magnification!

Building water rockets is a little more complicated. I found some sites on the web, but everything was a lot more complicated than anything I ever did with my kids. Take a 2-liter bottle and buy a cork to fit the top of the bottle very snuggly. Push a ball valve needle (for blowing up playground balls) through the cork; make sure the needle goes all the way through so that the air hole will be in the bottle when the cork is in place. Build a small 3 or 4 legged stand with a hole in the middle that is small enough that the bottle can nest upside down in it securely, but not so that it's wedged in (your son can have some fun with hammer and nails, too, under your supervision). Fill the bottle about 1/3 to 1/2 full with water (you and your son can experiment with how much water works best). Place the cork tightly in the top of the bottle. Hold your finger over the end of the valve and turn the cork side down in the frame. Quickly attach a bicycle pump to the valve (a foot pump works well) and start pumping! Make sure that no one is leaning over the bottle. When the pressure gets high enough, the cork will pop and the bottle will go flying and spray everyone with water as it goes -- definintely NOT and indoor activity! :-) We've gotten bottle to fly over 50 feet.

Also, a small safety thermometer, funnels, a scale, measuring spoons and cups, a balance scale and almost anything you have around the house can be turned used for a science project. Want to build a Bernoulli blower? All you need is a blow drier, a soccer cone (open at both ends) and a beach ball. I would go looking for a good book on science experiments for preschoolers at the library or book store. Take your son to the CA Science Center and the Kidspace museum in Pasadena for hands-on fun and ideas. Visit the aquarium and the zoo. (I would wait to go to Griffith Observatory for a few years).

Most importantly, encourage your son to constantly ask questions and try to find a way to answer them for himself (at his level of understanding, of course). Why does this happen when I do X? What happens if I do Y instead -- how does that change it? When will A happen if I do B? What can I do to make something work better? First and foremost, have fun!