Lemons contain citric acid, and this allows different metals placed in them to become electrodes. To make a lemon battery, you'll need a copper penny and a galvanized nail. The copper will become the positive electrode, and the nail will become the negative electrode. Insert the penny into one side of the lemon and the nail into the other side. Using a volt meter -- a common hardware store item -- show your child how to measure the amount of voltage coming out of the lemon.
This experiment can be expanded to continue the learning lesson. You can choose to daisy-chain several lemons together to show how voltage can increase, and once done, you can attach miniature jumper cables to the lemons via the electrodes and try to light an LED (Light Emitting Diode). An LED can be found at specialty electronics stores.
Lemons and limes may sit next to each other in the grocery store, but that doesn't mean that they're the same in all aspects but flavor. This simple experiment will help your child with deductive reasoning skills and with an understanding of density.
Gather a few lemons and limes, and find a clear bucket or bowl. Fill the bowl with water and place the lemons and limes inside. Most of the time, the limes will sink and the lemons will float. Let your child deduce the reason for this: Is it weight, the skin, density?
Allowing them to do the research increases their information-gathering skills, deductive reasoning skills, and several different science skills. Should your child need a little help in the right direction, the generally agreed upon reason that limes sink is that they have a higher density than lemons and the water in the tank.
When the other experiments are over, or if you've used a lemon or lime for juicing and have seeds leftover, this is a perfect experiment to learn about photosynthesis and the plant cycle. Wash the seeds to remove the sugars and any leftover lemon pieces. The leftovers can cause fungus to attack the seedling in its youth and kill it. Plant one seed each in two pots filled with soil that is acidic -- this can be done by adding coffee grounds to the soil.
After the seedlings have broken the surface of the soil, place one in direct sunlight, and leave the other plant inside a dimly lit part of the house. Let your child measure the growth rates for several days and then switch the plants. This will teach your children about photosynthesis, plant growth and the power of the sun.
The expression "shiny new penny" can be lost on many of us because all of our pocket change is tarnished. This final experiment allows your children to get a little messy to clean things up. Ask your children to try and make your spare change shiny again.
The first place most children start is with soap and water, and while this will help with dirt and grime, it won't help with tarnishing. Have them look for other substances they think will remove the tarnish, or have a few items picked for them to choose from, such as vinegar or baking soda. When you're ready to teach them about acidity and how it can remove tarnish, juice the lemons and limes and use the juice to clean the coins.