It's traditional for men to do it if passed by a hearse or when discussing serious illness or disasters.

His lawyers said he had a "compulsive, involuntary movement" because of uncomfortable overalls.
But the court ruled his behaviour was an "act contrary to public decency" and said the law "required everyone to abstain from conduct that is potentially offensive to collectively held feelings of decorum".
The man was fined £1520 and ordered to pay £760 in costs, reports the Daily Telegraph.
Judges pointed out that if men needed to grab their crotches, they should wait until they were in the privacy of their own home.
It is now understood that a local group of school teachers have cancelled their sky trip to Italy after hearing this news, and will be going to Poland instead.