I read somewhere that when you roast Chestnuts, you only need to poke their shells with a fork.

This is what raw Chestnuts look like. They are inside of their outer shells. The picture was taken by, Benjamin Gimmel, Wikipedia Commons.

640px-Frucht_der_Edelkastanie Bejamin Gimmel WCThis is a picture is taken by, Peacher, German Wikipedia.

640px-American_Chestnut Peacher de wikiI will use WC for Wikipedia Commons.

This is a picture of Chestnut trees  by, Schmzo WC.

640px-Castanea_dentata Schzmo WCA lone Chestnut Tree  by, Jaknouse, WC.

455px-Castanea_dentata-field_trial2009 Jaknouse WCHere is a picture of raw Chestnuts that were shelled  by, Douglas P. Perkins, WC.

640px-Bowl_of_chestnuts Douglas P Perkins WCA picture of baked Chestnuts  by, Maderibeza, WC.

640px-Kestaneler Maderibeza WCWhen water is heated it expands. This drawing gives an idea of this.

heat waterWhen I poked the Chestnuts with a fork and put them in the oven, it wasn’t long before they exploded. The water in the Chestnuts expanded and them broke the shells. When water gets hot enough, it becomes steam. When it gets cold enough, it becomes ice.
Before this I had always sliced the flat part of the shell with a knife. I went back to this after the experiment. I’d get someone who knows how to slice the Chestnuts. We have to be very careful around knives. This is just a little warning I feel the need to give.

Two example of expansion. The first is railroad tracks that contract in the cold and expand in the heat. They buckle. The picture is by, US Government.

Rail_buckle USGOVThis last picture is of the steam phase of Castle Geyser in Yellowstone National Park  by, Brocken Inaglory, WC.

640px-Steam_phase_eruption_of_Castle_Geyser Brocken Inaglory WC