Today’s invention is a coat with variable thermal insulation. Sponges are great at containing static air, which in turn is great for holding in heat.
The idea is to make a number of sponges roughly in the shape of muscle groups in the torso (different coats would be needed for males and females as usual). These sponges would be placed in airtight (metallised) bags which would then be joined to form a liner for the coat. Each bag would have a simple valve (making them a bit like wine bags).
When wearing the coat on a cold day which warms up, the user simply presses on some combination of bags so as to squeeze the air from them and flatten the sponges. Each bag’s valve seals again and the coat has become much less insulating in a number of places -depending on personal preference.
When it gets cold again, the valves are squeezed briefly, allowing air to be sucked back in by the elasticity of the sponges themselves.