I'm sure many have you seen this, conducted the very same one, or have heard of it. But, I just wanted to share it since it is a personal favorite of mine.
What you need:
Empty Water Bottle
Pour about an about 2" of vinegar into the empty water bottle. Use a funnel or something small to pour in about 1-2 TBS. of baking soda into the deflated balloon. Then place the balloon onto the water bottle being careful not to tip any baking soda into the bottle yet.
After you have discussed ideas and made predictions with the class, you get to demonstrate your magic trick. Tip the balloon so the baking soda pours in.
The bottle starts to fizz, the kids get excited, you start to smile, and the balloon begins to inflate.
It's such a fun experiment!
Afterward the kids are all talking about what happened.
I capitalize on this and have them record what they observed in their science journals.
Other favorites of mine: Liquid Layers (Density) and Gas Takes Up Space(The tissue that doesn't get wet when it is in a cup in water).