Balloon Cottonbush / Gomphocarpus physocarpus

Does anyone happen to know what this tropical fruit is - maybe a couple of inches across? It was growing in the Conservatory at the Calgary Zoo, but I couldn't see a label near the tree. It looked so light and hollow, but I could be totally wrong - it was growing too high up to touch.

Thanks Rachel (Silver Pomegranate - my daughter) for the ID! This is also known as balloon cottonbush, bladderbush, wild cotton or paina-de-sada. Actually, I am a little confused as another species, Asclepias fruticosa syn. Gomphocarpus fruticosus, a species of milkweed that is native to South Africa and may lead to death in both livestock and humans, looks similar and I'm still not sure which of the two species my image shows. Just daren't spend any more time reading up about them - have been out all day and absolutely have to look through a handful of folders of photos to back-up and delete from my hard drive this evening before my computer doesn't have enough free space to operate!

www.botanicalgarden.ubc.ca/potd/2007/11/gomphocarpus_phys...