jk wrote:
Great ideas and suggestions, please do keep them coming. As several of
the ideas employ the magnification qualities of the lenses, how can I
figure out what the focal length of the lense is? Is is simply a matter
of holding the lense above a surface and moving it up and down until
maximum magnification/focus is reached and then measuring the distance
from the lense to the object? This would be useful to know in
designing, say the kids fun table.
thanks.
john k

To determine focal length, take a piece of white cardboard and hold it
behind the lens while pointing it towards a distant, bright object
(such as the moon or a streetlight at night). Move the carboard
forwards and backwards until the image is in focus. That would be the
focal length of the lens.

Speaking of which, you might make some kind of viewing box out of a
lens based on the setup mentioned above. You could use a translucent
film, or just plain paper, which could display an image. I would
caution you, though, to avoid this being pointed towards the sun
(unless of course you intend to make a solar powered water boiler :-)