First of all, if you haven't seen it, i'd recommend you stop reading this, go out and watch it. Stunning.

Got me thinking about the physics involved, so my opener for year ten this week was "How many balloons do you think it would take to lift you?"

Got the kids to make a few guesses (it was my birthday last week, so we actually had a helium balloon in the classroom - let the kids do a few experiments to test - that balloon could support a pencil and an eraser!)

Then got a few kids starting to ask the important questions:

how much helium can it hold?

how much could it lift?

we took it as a sphere, and found the volume of an average balloon (15cm diameter), converted this cubic cm to litres, before we hit "HowStuffWorks" and found out that helium has a lifting power of 1g / litre.

Now we're cooking!

changed our weights into g (we reckon about 50,000g average per kid?) and worked out how many balloons it would take.

Fun lesson.

Then one kid dropped the question that'd been bugging me.

How many balloons for the house in UP?

Google tells us that the average UK house weighs around 450t, and we work out that it'd take about 30 million balloons to lift one.

Pete Docter (Pixar): " we calculated it would take between 20 and 30 million balloons in real life, we used about 20,000 in the film"

How much do i love pixar that they would take that into consideration?!?!

We then went backwards and worked out how big the balloons would need to be to lift a house if 20,000 of them could lift a house (we think about 3.5m diameter). One of the kids tells me you can actually buy them this size from Army surplus stores in England.

Interesting! The problem i now have is that my kids are determined to buy a load of balloons and orbit the school (we have not considered the cost of helium to be fair...)