A reader writes:

Seventeen years ago my son was a three year old. We picked him up from nursery school one day and the director told us he was playing an interesting game with another youngster. He would climb on his buddy’s back, they would both scream “BOOM” and crash apart, apparently imitating the shuttle Challenger explosion. Today he is a senior aerospace engineering student. He called this morning from Houston, where he is on a college co-op assignment at Johnson Space Center this semester. It was our first word of today’s tragic news.