The backyard bottle rocket uses the well-known reaction of baking soda and vinegar to launch not 1, not 2, but 4 bottles into the air at once! With a relatively easy assembly time, and materials that wont break the bank, this family friendly project can be done within a single afternoon.

Step 1: Materials:

1- 2 liter bottle
3 (16.9 fl. oz) bottles
Duct Tape
Scissors
Vinegar
Baking soda
Paper towels
Corks
Safety goggles
Funnel (not required but reduces spilling)

Step 2: Part A, Pre-launch:

1.) Cut duct tape with scissors. Make it a big enough piece to fit around all 4 bottles together. (Approx. 12in.)

Step 3:

2.) Start off by taping one of the smaller bottles to a side of the 2-liter bottle (wrapping the tape all the way around the two bottles to ensure it's secure).

Step 4:

3.) Tape the other two bottles to the other sides of the 2 liter bottle.

Having the bottles spaced an equal distance from each other will improve the aerodynamics of the rocket.

Step 5:

4.) Wrap a big piece of tape across all four bottles to ensure it's stability.

Step 6: LAUNCH TIME!

1.) Find a location that is open and away from people! (Safety is paramount here)

Weather can play a crucial factor during launch.

Step 7:

2.) Fill the 2 liter bottle about half way up with vinegar.

To use all 'rockets', fill the 3 (16.9 fl. oz) bottles halfway with vinegar as well.

Step 8:

3.) Pour about 1/4 cup of baking soda onto a paper towel, and fold lengthwise (make sure it's secure and none of the baking soda spills out).

If it's a windy day completing this task inside may conserve baking soda from being blown away.
If you encounter trouble folding, rolling the paper towel around the baking soda into a thin roll and taping the ends shut will also work.
The 16.9 fl. oz. bottles only need half of 1/4 of a cup for each.

Step 9:

4.) When ready, carefully input the paper towel wrapped baking soda into the 2 liter bottle filled with vinegar.

It is highly recommended that if you're using all four rockets (The 2-liter and the three 16.9 fl. oz. bottles), you do not do this by yourself. Having 3 to 4 people will make adding in the baking soda to each rocket much more efficient and safe.

Step 10:

5.) Put the cork in place of the lid of the 2-liter, give it a shake and place the rocket downwards (cork side should touch the ground).

Safety is paramount here. The person who is putting the baking soda into the bottle, shaking it, and flipping it over should be wearing safety goggles at ALL times! An adult should be the one tasked with this part of the project.
The shaking just decreases the time it takes for the reaction to occur. If you were to flip it upside down with no shaking, the rocket would still launch within 15-30 seconds.
When using all four rockets, complete this task in a synchronized manner (As such all participants adding the baking soda should be wearing safety goggles).