ストッパーを閉じたまま、熱を加え続けたら、容器は破裂したはずです。逆に、蒸気が水に戻ったら、容器はこうしてつぶれます。蒸気が凝集したので中が真空になったからです。

If I had closed the stopper, and still kept the heat applied to it, it would have burst the vessel; yet, when the steam returns to the state of water, the vessel collapses, there being a vacuum produced inside by the condensation of the steam. - Michael Faraday『ロウソクの科学』

ゆったりとしていてゴムがないため、かぶれを回避できるなどの理由からアトピー性皮膚炎を患う患者や、ブリーフのように体に密着せず通気性があることが高温多湿な日本の風土にも合っていて身体に良く、精巣の温度を適度に保ち精子数を増加させる効能もあるらしいとの評判から不妊に悩む男性に利用されているようだ。

For example, Ecchu fundoshi seem to be utilized by the patients who suffer from atopic dermatitis because a rash can be averted thanks to their looseness and lack of elastic, and by the men who suffer from sterility based on the rumor that because they don't adhere to the body and they breathe well, they are suitable to the natural features of Japan with high temperature and high humidity and good for the body, and that they have the effectiveness to keep the temperature of the testis moderately and increase the number of sperms. - Wikipedia日英京都関連文書対訳コーパス