Capillary attraction is the way water clings to certain objects. You do not need to go into detail with your preschoolers. Just tell them you are going to study the way water moves and will use science to "feed" a plant. Put the preschoolers into pairs and give them each a plant that they have to take care of. Flower plants are best as the results are more obvious. Give each student a small container and ask them to fill it with water. Put the container of water above each pot plant. Now hand them each a piece of cotton yarn. Ask them to put one end in the water and bury the other end in the plant soil. Leave the plants in a shady spot for about a month (topping up the water when needed) and then go back with the students to check on them. You should find a group of living plants and empty glasses.
Show students pictures of stalagmites and stalactites and tell them they are going to grow their own. Heat 2 cups of water in a saucepan, making sure not to boil it. (Obviously, you should do this, though students can help if you wish.) Slowly pour Epsom salts into the water. Explain to students that the water can absorb the salt for a while but that eventually it will not be able to "hold" any more. As you do so, pour in more salt until the water stops absorbing and the salt sits at the bottom. Show this to your students. Pour the salty water into two cups or containers. Now take a piece of cotton yarn and tie paperclips at both ends to act as anchors. Put one end of the string in each container, making sure it is in the water. The yarn should hang down a little between the two containers. After a few days, stalactites and stalagmites will form and your students should be amazed.
This is a fun experiment, and the children can even eat it when they are finished. As with the previous experiment, heat some water, but this time dissolve sugar into it. Wait until the water cannot absorb any more sugar. Allow the water to cool and then pour the solution into one container for each student in the class. Get the students to tie a piece of yarn string around the center of an ice cream stick or plastic knife. Tell them to lay the stick across the top of the container and hang the string down into the water. It should hang about halfway into the water. If the string is too long, help them cut it. Take several different colors of food dye and tell students to add a drop of two of whatever color they like. Put the containers in a protected area where they will not get bumped. Take them out one week later and show the students. Sugar crystals will have formed all the way up the string and onto the stick. This experiment will show the students how sugar crystals grow and form. They can eat the sugar if they wish or take it home to their families.
This is a very simple experiment which can be used to explain magnets and the earth's magnetic field. Hand a small bar magnet to each of your students. Mark on each of these magnets the North and South Poles. Ask students to tie their yarn string to the middle of the magnet and to hang it from a stick or pencil. Now tell each student to spin the magnet as much as they can. After about half a minute, tell them all to stop let the magnets come to a rest. As each magnet stops swinging, they will all end up "pointing" the same way (with the poles in the same position). Tell your students that all the magnets are aligned with the earth's magnetic field. Use this experiment to launch a class on earth and magnetic fields.