Everyone knows laughing and/or crying are both great healers.
But it's always a hard thing to know how much time you need to let pass before being able to laugh about something.
Jimmy Carr may have discovered it's always too soon to joke about the 9/11 terrorist attacks this week.
While appearing on Comedy Central's Roast of Rob Lowe (which aired in the UK on the 15-year anniversary of the tragedy), Carr made a joke about the terrorist attacks that seemed to go a step too far.
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He shocked the audience by making reference to Saturday Night Live comedian Peter Davidson, whose father died during the 2001 attacks.
"I'm appalled that people would come here and make jokes about the sacrifice Pete's heroic father made on 9/11," Carr said. "This is not the roast of Pete Davidson's father.
"That was in 2001."
Rob Lowe was shown mouthing the words "wow, wow", while Davidson smiled uneasily.
Carr has a history of crass humour.
In 2009, he received criticism from several newspapers for a joke he made about British soldiers who had lost limbs in Iraq and Afghanistan, saying that the UK would have a strong team in the London 2012 Paralympic Games.
He backed his own joke by saying it was "totally acceptable" in an interview with The Guardian.