This is a neat experiment which kids often find quite exciting, particularly in a large room (preferably away from any outside doors on windy days, even small amounts of wind are a pain for this one). It's best when combined with the vacuum demonstrations (which show that air is not massless).

Bottle with balloon

I usually start by asking the kids what is in the bottle (lots of kids will say nothing, this experiment is a good way of seeing that air isn't nothing!), and what will happen if we heat it up (the balloon expands). Depending on how old they are and how much they know you can talk about the following while you do the heating:

• Air is a gas (some children will be confused by this as they think it's a mixture of different gases which is not relevant here, but can be something to talk about if they're interested).
• Gases usually expand when they get hot.
• What this means in terms of particles, ie they gain energy, move faster so fill up more space. A reasonable analogy is that of a class of children all walking round past each other needing less space than the equivalent class running.

Heating is best done by running it under a hot tap, then you can cool under the cold tap, but you can use the toaster if you don't have a sink (for obvious reasons don't do a mixture of the two!). If using the toaster, you need to hold it a good distance away to avoid melting the bottle.

Balloons

I sometimes get kids to think of something powered by hot air (many get it straight away, some need a remarkable amount of prompting and loads say aeroplane...).

To make a balloon fly:

• Put the cardboard shield around the toaster, it's easiest if you turn it on first.
• Put the bag over the shield, pull it all of the way down to the bottom.
• Let the air inside heat up and off it goes.

You can get sensible kids to do this themselves.

Things to talk about include:

• Where does the air go when it expands? It fills up the bag then escapes out the bottom. This makes the bag lighter, so it floats up to the ceiling. If they all say "it rises", you can explain that we're trying to understand why it rises, or hold the bag down and say that it can't be rising because there's a bag in the way!
• Why do we have the bits of tape on the bottom? They are weights helping the bag go up in a straight line, if it gets tipped over sideways a bit then the weight pull it back straight again (hold the bag over your arm and tip it to demonstrate this). Some kids will know that real hot air balloons carry sand bags as ballast, this is more to help them control their height than to stop tipping, the weight of the basket/passengers is enough for this. You can try launching a bag without any weights, you should find that it tips over and falls back down quite quickly, though you need a relatively high ceiling to see this.
• How can we make it go higher/faster? Heat up the air more at the beginning by holding down the bag. Be a bit careful doing this, you can melt the plastic bag quite easily and it all gets quite hot. Different sizes of bag would be an option as well, though I don't think there are any in there at present.

(If you have the vacuum chamber as well, then you can use that to show that air does have some mass, so the bag does really get lighter when you lose some air, this will surprise most children and their parents. With good groups I think this is best done after the balloons, though less attentive children may lose interest once they've seen the exciting bit)