The number 11 and the Twin Towers are represented by two matches: in English the Italian word “fiammiferi” means matches and it reminds us of the word “effimero” (ephemeral) and the concept is connected to the fragility of the human condition, like the event of the Twin Towers, and the importance of the Ones we love "here and now".

"What does that mean--'ephemeral'?"(…)
"It means, 'which is in danger of speedy disappearance.'"
(The Little Prince by Antoine de Saint Exupery)