Let each child hold a lemon and tell you what color, texture, size and shape the fruit is. See if the lemons will float by getting a bucket of water and letting the children put the lemons inside. Ask them to guess how many seeds are in a lemon, then cut one open and count them together. If the children want to, let them taste a segment of the lemon and describe the taste.
Give each child a piece of white paper, a cotton ball or cotton swab and some fresh lemon juice. Let the children draw a picture using the cotton to draw with the lemon juice on the paper. They won't be able to see anything. Then set the papers out in the sun to dry. In a short time, the lemon juice will turn brown from the heat, and the children will be able to see their drawings.
Get a large soda bottle and fill it partway with a mixture of water and lemon juice. Add a small amount of baking soda. Your preschooler will love watching it fizz up and nearly, or completely, overflow. Once the bubbles have subsided, add a little more baking soda and quickly fit a balloon over the top of the bottle. The gas released from the chemical reaction will inflate the balloon.
Give each child a whole lemon and a stick and ask the students to push the lemons with the stick for a short distance on a level surface. Although the lemons are almost round, they are slightly elongated with bumpy ends, and the lemons will roll in all sorts of directions. The children can work on their coordination while trying to get their lemon past the finish line.