In order to extract DNA from cells, we have to burst the cells open. A DNA extraction solution needs three parts: distilled water, something to burst the cell membranes and something to help precipitate the other material out of solution. In a classroom, you can use more precise materials, but for these purposes, we can use distilled water, liquid dish soap and table salt. Take 1 gallon of distilled water and pour out a little to make room for the other components. Add 1 cup of the liquid dish soap and 1 cup of table salt. Shake this up and put the concoction aside for a moment.
Later on, you will need some extremely cold isopropyl alcohol (regular rubbing alcohol) so go ahead and place some in the freezer now. Decide on your DNA source. Depending on how inquisitive your burgeoning scientist is, you might want to do this with a variety of sources. Anything that is living has DNA, but some of the best sources for this experiment are fruit, especially strawberries and kiwis. To prepare for the DNA extraction, chop up the fruit or vegetable into manageable pieces and place in a plastic zipper bag. If you want to extract your child's DNA, have them swish a small amount of water in their mouth for one to two minutes and have them spit into a glass. The swishing should pick up enough of their cheek cells so they can view their own DNA.
Pour enough of the DNA extraction solution into the plastic bags to wet the fruit but do not fill the bag up. Seal the bags and expel most of the air and have your child mash the fruit with their hands in the bag. Once the fruit is thoroughly destroyed, place a coffee filter over a glass and strain out the leftover pulp so the liquid goes to the bottom. If you are doing your child's mouthwash DNA, add DNA extraction solution equal to about 1/2 of the volume that is already in the glass (this doesn't have to be too precise to work). Stir it up and set aside.
In the glass in front of you, the DNA is already extracted and ready to be visualized but it is mixed up with all the other cell parts. This is where the isopropyl alcohol comes in, and the colder it is, the better this will work. First, find the smallest, thinnest glass or clear plastic container you have and fill it about one-third of the way with your DNA solution. You can do this in a large glass but you are going to need more alcohol and the small amounts of DNA will be harder to see. Once you have your solution ready, use an eyedropper or baster to slowly add the alcohol down the side of the glass. Be careful not to pour it in because the alcohol and water will mix and you will not be able to see the DNA. If you do this correctly, you should create a layer of isopropyl alcohol over the extraction solution.
Almost immediately as the alcohol is added, you will see a long, whitish precipitate form at the meeting of the two layers. This is the DNA. It is stringy and you can play with it with a toothpick. If the strands don't appear, gently agitate the bottom layer to allow the DNA to float to the barrier. You can place your child's DNA or the fruit's DNA in another glass with some distilled water and add some food coloring or whatever other ideas you may have. Be sure to talk to your child about why DNA is important and repeat this experiment with any other source your child can think of. They might not all work but at least you can show them that science can be fun.