Make paper cup telephones with preschoolers for a science fair project. Help them to poke a hole in the bottom of each paper cup with a nail. Insert a long piece of yarn or string through the outside bottom of one cup through to the inside of the cup and tie it to a paper clip. Do the same with the other end of the string using the other paper cup.
Set up an area at the science fair where preschoolers can make finger paint artwork to show how two colors mixed together make another one. Let them combine blue and red to get purple, yellow and blue to get green and red and yellow to get orange. Display their artwork or let them take it home with them.
Blow up balloons with preschoolers at a science fair in order to show them what happens when an open balloon is released or when they rub blown up balloons on their hair. Use balloons that are easy to blow up -- typically large ones -- and let them release them into the air or rub them on their heads to show how static is created.
Let preschoolers examine a variety of flowers to see what their similarities and differences are. Have them count how many petals each flower has, examine the stem and look at flower colors. Guide them in their observations by asking questions and encouraging them to make comments about what they see.