A sponge!
or
We make perfect cents.
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One's a spineless, poisonous blob. The other is a form of sea life.
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1,000,001: One to change the bulb and 1,000,000 to rebuild civilization to the point where they need light bulbs again.
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When it came time to pay, the skunk didn't have a scent, the deer didn't have a buck so they put the meal on the duck's bill.
or