Formulatedin1823,theMonroeDoctrineassertedthattheAmericaswerenolongeropentoEuropeancolonization.

ThefirstimportantexpositionintheUnitedStateswasheldinPhiladelphiain1876.

Someseaurchins海胆havevenomous(toxin)spines

ThefuturesurvivalofthebaldeagleisstillanimportantAmericanecologicalconcern.

Themegaphone(microphone)makesthevoicesoundlouderbecauseitpointssoundwavesinonedirectionandkeepsthemfromspreadingoutinalldirections.