By Michael Brooks STEFANO ZAPPERI probably doesn’t find the holiday season all that relaxing, but you have to forgive him. The sound of chestnuts roasting on an open fire or a Yule log burning just reminds him of his work. Then there’s grandma and her never-ending supply of sweets; listening to her unwrap them is no treat. Plus, of course, the holidays mean there are kids around the house, noisily scrunching up the wrapping paper from their presents. In fact, the best gift you could buy Zapperi might be a sturdy set of earplugs. Zapperi’s problem is that he is a leading light in the study of crackling noise. Roughly speaking, crackling is a series of snaps or pops of many different intensities. It has a distinctive property: there is no way to predict how loud the next burst will be or when it is coming. But a paper published in October by Zapperi’s group at La Sapienza University in Rome, Italy, sheds new light on the physics of crackling and might even raise hopes that the holidays could get a tad quieter in the years to come. All sorts of things crackle, from clusters of firing neurons to Earth’s tectonic plates, and if we knew why, it might aid our understanding of these processes. But just as interesting is the fact that some things don’t. Why is it that log fires crackle, but popcorn just pops? Why does scrunching up gift wrap make a din when doing the same thing to silk doesn’t? And – the $64,000 question – why do wrappers and snack-food bags make such a racket?