Contact is the one that actually ruins things

Two soft plastics stored touching each other for months do not stay two separate objects. They exchange plasticiser at the boundary, and the exchange is not symmetrical: it runs from the softer, more heavily plasticised material into the harder, less plasticised one.

In practice that means the cheap TPE thing you bought as a second option contaminates the expensive silicone one you keep next to it. The silicone picks up oil it did not have, goes tacky at the surface, and then you are reading a page about why silicone went tacky and concluding that the silicone was fake.

It was not fake. It was next to the wrong neighbour.

The rule that follows is simple and slightly annoying: soft plastic touches nothing soft for long periods. Not another toy, not the vinyl window on the packaging, not a PVC-lined pouch, not a plastic bag that has been in a warm drawer for a year.

Silicone against silicone is fine, because silicone has nothing to migrate. Everything else needs air around it.

Heat, and why the bathroom is the worst room in the house

Plasticiser migration is a diffusion process, and diffusion roughly doubles for every 10°C. A drawer at 30°C ages a soft toy several times faster than one at 18°C, and it does it silently.

The bathroom cabinet is the popular choice and the worst one, because it combines the two conditions that matter: it is the warmest room in most British flats for part of the day, and it is the dampest. Warm plus damp is the exact environment for both plasticiser loss and the smell problem that comes from storing something with residual moisture.

A drawer above a radiator has the same issue in winter. So does a windowsill, which adds sunlight on top.

Sunlight does something different

Ultraviolet does not just accelerate the additives leaving. It attacks the polymer chain itself, which is why sun-damaged plastic yellows, then chalks, then cracks. That damage is structural and does not come back.

This matters less for something kept in a drawer and more for anything that lives on a shelf near a window, which is exactly where people put the nice-looking ones.

Moisture, and the sealed-bag mistake

Anything porous, and anything with a seam and a motor, holds water after washing in places a towel cannot reach. Putting that into a sealed bag traps the water in with it.

That is why "store it in a ziplock" is bad advice for anything that is not bone dry. A sealed container is fine once the item is dry. It is a mould incubator before that.

For silicone with no motor, this is easy: wash, dry with a towel, leave it out for an hour, then it can go anywhere. For anything with a motor, a seam, or a porous surface, give it a day in open air before it goes into a closed box.

What actually works

• Separate compartments, or separate pouches. A bigger box does not help; the point is distance, not volume.
• Breathable over sealed for anything that might still be damp: cotton or a woven pouch, not plastic.
• Room temperature, dark, dry. A wardrobe shelf is better than a bathroom cabinet, and a bedside drawer is fine if it is not above a radiator.
• The original box is usually good if it is rigid with a foam insert, and usually bad if it is the type with a moulded plastic window, which is often PVC.
• Not the drawer with the hair dryer, the phone charger and the cheap earbuds. Those cables and cases are soft PVC more often than people realise.

A note on pouches

The satin drawstring bag in the box is fine if it is actually fabric. A lot of them are a polyester weave with a plastic coating on the inside, which puts you back in the contact problem.

The test takes ten seconds: turn it inside out and look at the inside surface. If it is shiny and continuous rather than woven, it is a plastic film, and it will do what plastic film does.

Discretion, which is a storage problem too

If you share a flat, or have children in the house, the storage decision is also the "do I actually own this" decision. Something that lives in a box you would not want opened tends not to get used, and something that gets used is the only reason to have bought it.

A lockable box is not paranoid. It is the difference between owning one and not.