Some of these crown cap answers seem to conflict with my childhood  memories. 
When in elementary school (approximately 1958), kids used to collect  bottle 
caps to flick in the air (snapped fingers sent the cap flying like a  Frisbee 
today). The top flickers sent the caps the furthest or closest to lines  drawn 
in the sand. I recall experimenting by putting pennies in wax to lend  weight 
to the flick. Those caps at that time were cork. At that time, my family  
engaged in hotelier work and the bottle caps with foil were mixers and tonics  
from the bar. Maybe I was just observing the transition point in technology. In  
truth, I don't think I ever gave bottle caps much thought after 1959. This 
would  place west coast bottle caps with cork liners a tad later than the 1955  
rule.
 
Ron May
Legacy 106, Inc.