Mochi (Japanese: 餅, もち) is Japanese rice cake made of glutinous rice (Mochigome).
The rice is pounded into paste and mould into the desired shape.
In Japan it is traditionally made in a ceremony called Mochitsuki (mochi-pounding).
Hit the rice dough with a giant pestle-like baton and the other to pull it flat with their hands in between blows.
This is done with gusto and takes great skill on the part of the dough-puller not to get their fingers smashed in.
Here's a vide of mochitsuki by a Japanese fastest Mochi maker.