In New York and Afghanistan, Sierra told children to draw whatever they want. But many children in New York chose to draw the collapse of the World Trade Center towers while those in Afghanistan drew towns ruined by the U.S.-led military attacks on the country or their family members being killed by the Taliban, Sierra said.

Many of the children used black, gray and dark brown crayons, the "colors of sadness," to draw their pictures, just as many children did in other troubled countries, said Sierra, who has also gone to Kosovo and Chechnya to hold art workshops.

But Sierra says he knows from his experience that putting terrible incidents into pictures can help children release their feelings.

"Some kids say 'I don't want to draw,' and it's fine. But, in general, children want to draw. They use (drawing) to express what they want to express. They took the chance to say what they wanted to say. Because children cannot put their emotions into words, the only way is to draw."

"Some people would get depressed from drawing a picture of war, as they remember the war, but after that we do origami and have fun together. I think that process is cathartic. I think the function of art is to provide catharsis. Art is about purifying souls through the process. So when a child is drawing war scenes, he also goes through that process. After drawing war scenes, he smiles to me."