On field trips, undergrad students sometimes get the bright idea of salting
an outcrop with a conspicuous fossil that could not have occurred there.
What they expect is to see a flustered professor trying to make sense of
the impossible, like a trilobite in a Cenozoic outcrop. What they get is a
professor saying, "Oh, yes, this doesn't fit here. Someone must have
dropped it here after visiting another outcrop." This is a politer
explanation that does fit occasionally, when a collector's backpack gets
too heavy. It's bad form to contaminate an outcrop, but inexperienced
collectors do it sometimes. The professor usually knows perfectly well who
salted the outcrop--it's the mischievous student who is lurking nearby with
gleaming eyes--but it's better to turn the joke back on the student this
way. Later on the bus, the student will ask his friends, "How did he KNOW
that the trilobite didn't come from here? He must be really good!"
 
Which is just what the doctor wanted.
 
Andrew K. Rindsberg
Geological Survey of Alabama
Tuscaloosa, Alabama, USA