Even though magnets do not stick to all stainless steel surfaces, all stainless steels will have some small degree of ferromagnetism (magnets stick to them). The degree to which they do depends on nickel content relative to the other materials in the alloy. We have chosen 300 series stainless steel for the tray of our lunchbox and 400 series for the lid. The 400 series has slightly less nickel content which makes it better suited to holding the magnets. 400 series is somewhat hard to work with though, so it is not well suited to the lower tray of the PlanetBox assembly due to the deep pockets we form in the sheet material. For the tray, as well as the Dippers, we use the 300 series material which has very good workability properties. Both materials are highly corrosion resistant and completely food safe.