If you mean the viral aluminum-foil ball “hack”, there are actually a couple of different versions.
The classic aluminum-foil ball
The idea is simply to compress aluminum foil into a very dense ball. The Japanese version that went viral was made into a polished metal-like sphere by repeatedly hammering, sanding, and polishing it.
What you need
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1 large roll of ordinary aluminum foil
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A sturdy surface
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Rubber gloves
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Hammer or mallet
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Optional: waterproof sandpaper, roughly 100–2000 grit
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Aluminum polish and a cloth if you want the mirror finish
Method
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Roll the foil tightly into a rough sphere.
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Compress it by hand as much as possible.
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Put it on a sturdy surface and tap/hammer it repeatedly, rotating the ball constantly.
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Keep working around the entire surface. The ball will become progressively smaller and considerably denser.
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When you have a reasonably smooth sphere, start with coarse sandpaper and gradually move to finer grades.
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Finish with aluminum polish and a clean cloth for the shiny appearance.
A documented version used a large roll of foil and progressively compacted the ball with mallets, then polished it.
But if you literally carry one in your bag…
If you’re carrying it because someone told you it has a special practical benefit, that’s a different claim. Aluminum-foil balls are commonly promoted for reducing static in laundry because aluminum conducts electricity, but testing has found the effect inconsistent; they’re not some miraculous all-purpose invention.
And don’t put a foil ball in a microwave—metal and microwaves are a dangerous combination.
If you tell me what your friend said the ball was supposed to do in your bag, I can explain the exact trick and whether there’s any science behind it.