Why buy an ant farm? It's simple and fun to make one from a few items you have lying around the house.
WARNING: Do not put fire ants in an ant farm!
Cut the mouths and necks off both bottles. Wrap the outside of the smaller bottle in black construction paper, and secure the paper in place with tape. Put Blu-Tack on the bottom of the wrapped bottle, and place it in the middle of the larger one.
Pour the dirt or sand through the funnel into the space between the containers. Stop about a half inch before you reach the top of the 1-liter bottle.
Decide what type of ants to keep. The best source is your own backyard. If you can't find suitable ants, order some from a mail-order supplier.
Put on the gloves and use a shovel to transfer part of an anthill to a jar, or place the jar near the anthill with a bit of fruit inside. Capture 50 to 80 ants, and try to capture a queen; without one, your farm will only last a couple of months. The queen is larger and longer than the other ants, and will be surrounded by many other ants.
Wear gloves: Ants will bite to protect their queen.
Pour the ants from the jar into the soil in the 2-liter bottle, and quickly cover the top with a paper towel or old nylons. Secure the lid with rubber bands. Poke air holes in the larger container's cover with a pin or needle, making sure they are too small for the ants to crawl through.
When you're not looking at the ants, cover the outside of the farm with another piece of black construction paper, secured with tape, to simulate an underground environment. Don't keep the farm in direct sunlight.
Once a day, soak a cotton ball in water, take the cover off, place the cotton on top of the dirt, and cover the farm back up, allowing the ants to get drinking water. Feed ants small pieces of fruit and breadcrumbs dipped in sugar water or honey. A teaspoon of food is enough for 20 to 40 ants every two or three days.