1) Ordinary - ie small - matchboxes only, ie none of them huge great boxes of 'kitchen matches' like my mum has in her kitchen
EDIT BY TREVIRA (sorry!): there seems to be some confusion about ordinary or standard sized matchboxes. So here's a suggestion - 52mm long x 37mm wide x 17mm deep approximately, with an allowance of a few mm either way.
2) No duplicates. Only one example of each object will be counted. So you can fill yer box with salt, but it will only count as 1 grain.
3) [Added 09-Feb-2009] The fully loaded box must slide into the outer sleeve without causing a bulge. Please include a pic in your photostream to demonstrate this.
ORIGIN:
Kollage Kid reminisced and Trevira ran with it, here: