Can anyone explain why 0! (zero factorial) = 1 ?? ?

zero factorial = 1 according to my calculator.. i was under the impression factorial was.... like 5! = 5x4x3x2x1 so why is 0! ie 0x0x0x0x0 = 1 ?? can anyone explain this? It's not homework, so don't tell me to 'do my own homework' and i asked my math teacher... but she said she couldn't... show more zero factorial = 1 according to my calculator.. i was under the impression factorial was.... like 5! = 5x4x3x2x1
so why is 0! ie 0x0x0x0x0 = 1 ??

can anyone explain this? It's not homework, so don't tell me to 'do my own homework' and i asked my math teacher... but she said she couldn't remember, as this chapter was new on the course... she said she'd look it up... but she'll forget....
so anyway.. can you explain it??

thanks! =]
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