Blowing up one balloon inside another can be a little fiddly to start with, but with practice creates an intriguing effect that will delight kids and adults. Once you understand how to put one balloon inside another, you can progress to putting several small balloons inside one big one. Make a pinata-type game for a kids' party by having one small, inner balloon for each guest. Put a message inside each balloon that details a prize won, or simply make it fortune cookie-type message. Use helium to inflate the balloons if you want them to float, or make the job quicker by using a balloon pump if you have a lot of balloons to inflate.
Push the dowel inside the neck of the balloon that will go inside the other one. Set this aside.
Inflate the balloon you will use as the outer balloon, but don't tie it off.
Push the balloon with the dowel inside the inflated balloon, leaving just the neck hanging outside. Don't allow all the air to escape from the inflated balloon but don't worry if it deflates slightly.
Take the dowel out of the inner balloon, then inflate it to the size you'd like.Once you have a little air in the inner balloon, this will help to plug the neck of the outer balloon to prevent the air escaping. Tie a knot in the neck of the inner balloon and push it completely inside the outer balloon.
Re-inflate the outer balloon if necessary, then tie a knot in the neck.
Use different coloured balloons, having a pale one as the outer with a brighter colour for in the inner balloon. You can use different size balloons or use two balloons of the same size and inflate the outer one more than the inner one.