My chemistry teacter did the same experiment, only he used condoms instead of a tin.
He filled two of them with the gas (to about 3 feet long!), and one of the girls in the class offered to hold the strings (they floated).

The whole class went outside to a school field. He attached a weight to one of the strings, and lit the first "balloon" via the technique of throwing a lit match at it.

The bang rattled the windows on the side of the school, this caused people in other classes to open the windows to see what the noise was. Imagine the scene they saw of a girl holding the string to a 3 foot long floating condom.

Suffice to say we had a large audience for the second explosion... =)

Jazz
(List 112, Mk2 12 gig #40. Mk1 4 gig #30. Mk3 1.6 16v)