Because it might crack up!
or
Only one, but she/he'll have to go out and buy the light bulb adaptor card first, which is extra.
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They charged one - and let the other one off.
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If one side has one, the other side has to get one. Once launched, they cannot be recalled. When they land, they screw up everything forever.
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The leaning tower of Pizza.
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"The way you make me feel, it really turns me on!"
or