Identifying the source or the type of these explosions is more difficult. In fact, one of the most frequent topics of conversation in Iraq is trying to figure out exactly what an explosion means. Is it incoming? Outgoing? A car bomb? Mortar? Or maybe just a cooking canister that exploded? The answer, too often, is that no one knows. Sometimes, on an embed with the U.S. military, a tooth-rattling explosion will go off nearby and soldiers will simply shrug and say it’s too difficult to pinpoint exactly what it is. Of course, some blasts are unmistakable. During one embed, the military lobbed artillery shells the size of TV sets at their targets. The explosions shook the ground and the trailer where journalists were filing their stories. At night, the shells cut an orange streak across the sky. According to the initial news reports, the same sort of artillery shells, probably 105mm rounds, were pounding targets in southern Baghdad this morning. Coming one day after an announcement that nine American soldiers had been killed in separate attacks, the shelling may have been intended as a message.

In military jargon, mortars and rockets, at least when they are incoming, are referred to as “indirect fire,” a somewhat misleading name. It refers to projectile weapons that travel indirectly between the shooter and the target, but such fire certainly doesn’t feel indirect when you’re on the receiving end. Two Americans, a soldier and a contractor, were killed by a rocket attack at the U.S. embassy in late March. Indirect fire can be deadly but it isn’t going to be a deciding factor in this conflict: the insurgents use it primarily as a harassment tool.

Even if no one can pinpoint exactly what a particular thud or boom is, it’s worth venturing a guess to assess the threat. For example, a loud boom strong enough to shake windows or doors is often a car bomb. A dull, deep sound, like the explosions this morning, can be outgoing artillery or aerial bombardment. The one sound most people dread, and is the most common, is the thump of mortars. If there is a crackling sound before the thump it’s probably an incoming rocket, which means time to dive for cover. Our staff usually throw on body armor and gather in a prearranged rally point. The one thing to remember with the rockets and mortars is that they’re rarely one-offs. They usually come in barrages of two or three. All of these explosions can make people a little jumpy. A colleague recently got rug burns on his chest from diving for cover. The threat? A backfiring car.