The professor wanted to see if the animals could tell the difference between the calming music and the music written to make them uneasy.

VOA: special.2009.09.29

"The nicest thing is you can still see their colors. There's enough contrast so that you can tell the difference between a red star and a blue star and show people that stars have colors.

VOA: special.2009.10.27

"In recent years,these companies have become much more sophisticated, up to the point where it's impossible to tell the difference between the counterfeit cigarettes and the actual cigarettes."

VOA: standard.2010.08.11