But where do they get the explosive from?
Glass is smashed on a car as Afghan soldiers block the scene after a suicide bomber's attack on January 12, 2011 in Kabul.
Drug trafficking is illegal, while arms trafficking also threatens national safety, as does trafficking in explosives, which is a criminal offence, because of the threat to innocent people and the possibility of igniting sectarian strife.
Investigations into the blast outside the Qiddissein (Two Saints) Church in Alexandria on New Year's Eve have revealed that it was caused by a bomb weighing 2kg, made of nails, ball bearings, TNT and other solid materials.
It killed 23 people and injured more than 90. This incident has been preceded by several similar incidents over the years.
Terrorism doesn't discriminate between Muslims or Christians, while its purpose is to destabilise society.
If you want to make explosives, there are thousands of websites to show you how. What makes it easier is that the materials are also available locally. Some of these websites contain videos that show you how to manufacture bombs.
Gunpowder, the first known explosive material, was invented around 1300. It consists of charcoal, sulphur and potassium nitrate.
Then, in 1867, dynamite was invented by Alfred Nobel. Most explosives contain nitro-glycerine, making them more destructive.
Such materials are sold in the local market, as they are needed by mine and quarry owners, says the former deputy head of State Security, Fouad Allam, adding that they buy explosives in huge quantities, some of which end up being sold for a lot of money on the black market, making certain people very rich.
He explains that trading in explosives has been made easier by the advent of the Internet, which, as described above, shows terrorists how to make them.
Talaat Mossalam, a strategic expert, told Al-Wafd opposition newspaper that some explosives are easy to make, but the more sophisticated ones are trickier.
Although the explosive materials used in big factories are carefully controlled, there is no control over the small workshops, some of which trade illegally in explosives.