Suppose you have a microSD card, how can you destroy it easily on almost no notice? You can be forced to reveal the key so encryption alone isn't enough.
Swallowing it won't work: http://www.thesmokinggun.com/documents/crime/giga-biter-obstruction-charge
A lighter is too obvious, and you may have trouble bringing that through checkpoints like airports. Ditto for keeping around some powerful acid.
Would it be easy to wire up a small device to hold the card and have a battery wired to the IO pins? Keep the card there and press the switch to overload it. Would that work?
What other ways?
I don't think you'd have too much trouble getting a book of matches through airport security. Just saying.