A container comprising: a container body that includes a wall portion separating inside and outside and is formed of a long-fiber-reinforced silicon-carbide composite material obtained by combining monofilaments of silicon carbide with a silicon carbide matrix, the wall portion having a thickness equal to a specific dimension; and a lid configured to close an opening of the container body, formed of a material containing at least silicon carbide, and equipped with such a wall portion separating inside and outside that thickness is within a range of 1 to 3 times the specific dimension.

A method of collapsing a collapsible box includes providing the collapsible box, the collapsible box comprising: a plurality of side panels including a first side panel and a second side panel, each of the plurality of side panels defining a side panel top edge; a box bottom; and a box top comprising a plurality of top panels including a first top panel extending from the side panel top edge of the first side panel and a second top panel extending from the side panel top edge of the second side panel, the second top panel comprising an overlapping portion; folding the first side panel towards the second side panel to confront the first side panel with the second side panel; and folding the overlapping portion of the second top panel over a top panel top edge of the first top panel.