Material: 2 Bottles same size and shape, 01 plastic lid (milk), large clear tape, hot Glue Gun, water, glitter, food coloring, sand, baby oil, sand, food coloring, etc.
Prepare the first bottle: fill 3/4of the bottle or less, depending what kind material you are going to use with water: baby oil , sand, glitter or only water. Prepare the second bottle: Make a hole at the milk lid, or any other plastic soft lid with desired size, and hot glue inside of of the bottle, wait until dry, then put more hot glue and place the bottle on top of the first bottle. Prepare the large tape to be use. Outside the two bottles, apply a good amount of hot glue, specially around the connection, then use the large tape around the the connection to seal the two bottles (give at least two rounds of tape), be really carful, the hot glue may melt the tape, press gentle the tape around the bottles connection. Wait a minute or two and check if its workings: Needs to be perfect on first time, then make sure you are using the right amount of glue before you finish.