Answer :

That's Chemistry...

But basically it's just the number of electrons 'shared' between two atoms.  A single covalent bond has one electron shared (e.g any diatomic bonding of the Group 7 elements)
Double covalent bonds have 2 electrons shared (e.g in the bonding of Carbon Dioxide)
Triple covalent bonds have 3 electrons shared (e.g in the bonding of diatomic Nitrogen)
Move covalent bonds make the molecule stronger 

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