Pour 1/2 cup of vinegar into an empty water bottle.
Place the funnel into the open end of the deflated balloon and pour in the baking soda.
Secure the open end of the balloon onto the top of the bottle, being careful not to dump the contents of the balloon into the bottle quite yet.
When you're ready, hold the balloon upright allowing the baking soda to fall into the bottle and mix with the vinegar.
Watch the ghost grow!