CHRIS BURNS, CNN CORRESPONDENT: Yes, Aaron. A day that people here will never forget, obviously. Over my shoulder, the candles still burning from mourners who left candles here on the steps of Johann ...

THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED. MILES O'BRIEN, CNN ANCHOR: Now for something really different, scientists are getting closer to understanding how ...