If you mean the “grandpa trick” of using boiling water, here’s the fuller explanation:
How it works
Weeds growing through driveway cracks are exposed and usually don’t have much protection from heat. When you pour freshly boiled water directly over the weed, the extreme heat damages the plant’s cell membranes and tissues. The leaves and stems can wilt rapidly, and the roots near the surface may also be damaged.
How to do it
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Boil a kettle or pot of water.
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Carefully carry it to the driveway.
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Slowly pour the hot water directly onto the weed, concentrating around the base and any visible growth.
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Let the area cool and dry.
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Check it over the following several days. Persistent or deep-rooted weeds may need another treatment.
Why people like this method
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No herbicide is required.
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It costs essentially nothing if you already have water and a way to heat it.
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It doesn’t leave a chemical residue.
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It’s particularly convenient for isolated weeds in pavement cracks.
The “0 effort” part is a bit of an exaggeration, though: you still have to boil and carry the water, and some weeds will regrow because their deeper roots survive.
Important: boiling water isn’t selective. It can kill or seriously damage nearby grass, flowers, moss, and other plants, so keep it confined to the crack or unwanted vegetation. Also take care not to splash yourself—the water can cause severe burns.
