Subject: Is there a mirror on the moon and which mission put it there?

I am an informal educator at a science museum.  Recently, while 
explaining echolocation to a student, I somehow got into how 
scientists have used the same basic idea about judging distances 
to objects that are hard to measure by bouncing signals off of 
the objects and measuring how long they takes to return.  I dimly 
recollected that astronauts had placed a mirror on the moon to 
reflect radio signals so slight fluctuations in the Earth-Moon 
distance could be monitored.  Of course, the kids got all excited 
and wanted to know more, but I can't remember much more about it 
(so I just veered the conversation smoothly back to 
echolocation:)).  Now I'm curious.  I looked through our library, 
but none of the books mention it at all, let alone details like 
how big it was, was it really a mirror or more of a reflecting 
dish, who put it there and are we still using it.  Can you help 
me out?  Thanks fo

Re: Is there a mirror on the moon and which mission put it there?