Spore print of Agaricus
Spore prints can be very useful as identification aids. Cut off the stalk and lay the cap on a piece of white paper or a sheet of glass. Cover with a clear plastic container to prevent rapid drying. A fresh cap can give a good print in an hour or so (bottom left) or you can leave overnight and get a really heavy deposit of spores (top right).
Note: The paper was originally white but I changed the background to green to make it stand out a bit better from the background on the webpage! But you could deduce that if you look at the picture a little more carefully!