Stainless beats bamboo for a truly plastic-free lunch box.

If you want a lunch box with literally no plastic parts, go stainless steel and skip bamboo.

Bamboo sounds natural, but most “bamboo” lunch boxes are bamboo fiber mixed with melamine resin — that’s plastic. Pure bamboo boxes are rare and they crack or warp with hot food. Stainless steel is the honest answer.

Best pick: PlanetBox. It’s 100% stainless steel — no plastic, no silicone, no gaskets. The lid closes with a magnet. It’s pricey at around $60, but it’s the only mainstream all-metal lunch box I know that’s also kid-proof. Made in Texas.

Budget pick: ECOlunchbox Classic. Also stainless steel; the lid just nests onto the base with a cotton band. Not as slick as PlanetBox, but it’s real steel and about half the price.

One catch: no plastic usually means no leak-proof seal. That’s physics. Don’t plan on soup or yogurt in these. Pack dry food — sandwiches, fruit, salad — and you’re fine.

The soup stays home.