If you mean the common banana-storage tip, the “one kitchen item” is usually plastic wrap.
Why it can help
Bananas naturally release ethylene gas, a plant hormone that speeds up ripening. The gas is released particularly around the stem/crown of the banana.
Wrapping the crown tightly with a small piece of plastic wrap can reduce the amount of ethylene escaping from that area and may slow ripening somewhat. This can help bananas stay yellow and firm for longer than leaving them together in a fruit bowl.
A few important details:
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Wrap the stems/crown, not the whole banana.
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Separate bananas if you want to slow ripening further.
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Keep them at room temperature while they’re still green or yellow.
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Once they’re ripe, refrigeration can extend their usable life, although the peel may turn dark.
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The claim that wrapping stems makes bananas last exactly 10 days longer is too specific; storage conditions, banana variety, ripeness, temperature, and handling all affect the result.
So the practical version of the tip is: cover the banana stems with plastic wrap rather than leaving the bunch exposed in a fruit bowl.
