I’m serious, these are perfect.  When I was little I used to use empty film canisters for the same purpose.  I don’t think those exist anymore, and these are better anyway since they are clear and I can see what’s inside.

I use these to collect all sorts of samples.  Water from ponds, moss, isopods, queen ants I come across, clay or dirt samples from the river, seeds from wildflowers, you name it.

They are small enough to have some in your pocket at all times, so you never miss an opportunity to collect a sample of something interesting to take home and examine under a microscope.  It makes every outing a possible adventure, and the kids are always excited to see what we might find, even when we’re hanging out in the back yard.