Custom Paper Soup Bowls.

To stop the lid coming off — which sounds backwards, so it is worth explaining.

Hot soup gives off steam continuously. Sealed inside a container, that steam has nowhere to go and pressure builds against the underside of the lid. Given twenty minutes in a delivery bag, that pressure can lift the lid off entirely.

A small vent lets the pressure equalise. Positioned and sized correctly it releases steam without allowing liquid out at normal carrying angles.

It also stops the opposite problem: a bowl sealed hot and then cooling creates a partial vacuum, which can make the lid genuinely difficult for a customer to remove.

Cold soups do not need venting — no steam, no pressure.

Not if the rim is right, and the rim is where you should focus when assessing samples.

The lid seals against the rolled rim, so any irregularity — a flat spot, a section slightly out of round, a crease from stacking — gives the lid a point to lift from. That is where essentially all leaks originate, not through the board.

Two practical checks: run a finger all the way round the rim of a sample and feel for irregularities, and press a lid on and try to rock it. A good pairing sits down evenly with no play.

The honest limitation is orientation. Even a well-made locking lid with a vent is designed to be carried upright. It resists spills at normal angles but it is not a sealed vessel, and no paper bowl will survive being laid flat in a bag.