To make colored "sand," grind a half-stick of colored chalk against sandpaper into a bowl, and mix in ¼ of salt. Make several colors, each in its own bowl. The total amount should be enough to fill the bottle.
Spoon layers of colored "sand" through a funnel into the bottle. (To make a funnel, poke a hole in a paper coffee filter.)
Use a blunt pencil to push designs into the layers.
Put the lid on the bottle, and add a ribbon, if you wish. (Do not use the salt on food after mixing it with chalk.)