This one takes a while but you'll get a gorgeous (and enormous) houseplant out of it. You won't grow any actual avocado fruits on the plant, but it will be leafy and glorious for up to two or three years - or compost it at the end of the summer and grow a new one next spring, if your house is as chilly as ours over the winter months!
Avocado seeds from supermarket fruit will germinate (eventually) - first soak in water overnight.
Then suspended over water - rounded side down - using 4 toothpick sticks poked into the stone, about halfway up.
It can take up to 8 weeks for the root to sprout, and during that time you need to keep the seed warm (i.e. at a 'normal inside of a house' temperature and out of drafts) and change the water twice a week.
Once a 2cm root has grown, plant the seed in a large-ish container of compost - you could use a regular flower pot or a cut-down 4 pint milk bottle / cut-down Tetra Pak, with holes made for drainage, sitting on a saucer. Make sure the seed is half buried and half exposed, and put it in a sunny, indoor position.
Water the planted seed enough to keep the soil damp (poke your finger down to where the base of the seed is to check the lower levels of soil are damp too).
You can probably guess what I'm going to say now - please don't go and buy an avocado specially, just to get at the seed. They are transported across the globe and no one wants to be doing an eco crafty activity which generates a huge carbon footprint! So, if you aren't an avocado-eating family normally, ask around - find a neighbour or friend who eats avocados, or if you notice avocado on the menu of a cafe ask them if you can have a leftover avocado stone or two.
These are really lovely to give or receive as presents - kids LOVE watching them grow, so I suggest germinating extra seeds and giving away the resulting baby houseplants as gifts. Enjoy!