Kagami-mochi, rice cake, is a traditional Japanese New Year decoration and a culture of offering mochi to the gods.
The reason for offering mochi is that the shape of the mochi resembles the shape of an ancient Japanese mirror.
In Japan, mirrors are said to be the three sacred treasures. Kagami-mochi is generally displayed until early January. Then eat this mochi. Modern commercially available kagami mochi are safe, but handmade mochi need to be taken care of to prevent mold from growing.
To make mochi, a pestle and mortar are used to pound mochi rice. If you don't have enough rice, you won't be able to beat it well, so use a certain amount of glutinous rice. Mochitsuki, pounding mochi, is mainly held around New Year's Day, and is an event where relatives get together.