Why two magnets
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The mechanism
Two magnets swallowed separately can end up in different loops of bowel. They attract through the tissue between them, trap it, and the pressure cuts off the blood supply to that tissue.
Which produces a perforation
Not a blockage. The wall between them dies and gives way, and the result is surgical rather than something that passes.
And it can be silent at first
Which is the part that makes it different from choking. A child who swallowed two magnets may be well for a while, so the event that needs urgent attention does not look urgent.
So the hazard is not the size
A magnet small enough to swallow is only dangerous because of what it does after. That is why the rule limits strength rather than only reaching for the size gauge.
Two is the number
A single magnet and a single metal object will do the same thing, which is why a magnet plus any swallowed ferrous part is the same injury.
Which bears directly on a modelling bench
A bench with loose neodymium magnets also has loose steel screws, pins and photoetch frets. Those are the pairs.
Keep them apart and keep them closed
Separate containers for magnets and for ferrous parts, both with lids, both away from the floor. That is the whole of the household control and it is a storage decision.
A lidded jar is not enough on its own
Magnets cling to the lid, to the rim, and to each other. Opening a jar of them over a bench is how they end up on the floor.
Which argues for a deeper container than you need
Headroom means you can get a hand in without tipping it, and a magnet stuck to the inside wall stays inside. 255 jars here.
And for counting them
Buying a hundred and knowing you have a hundred is the only way to notice that two are missing. In a house with small children that is worth the minute it takes.