May I refer you to the Ohlone Indians of Central California:
"Acorns were the most important food in the California Indian diet. Entire villages would move into the oak forests when acorn harvest time was near. The men would shake the trees, causing the acorns to fall onto the ground. Then the women and children would gather the acorns into baskets to be stored in a granary. Using a stone mortar and pestle, the women ground the acorns into flour. The mortar, a stone bowl, held the acorns. The pestle (a stick shaped rock) hit the acorns over and over again until they were ground into a fine powder. Acorns contain tannic acid which is very bitter and poisonous. The tannic acid had to be washed out of the flour before it could be eaten. This was done by carefully pouring water over the powder and letting the tannic acid slowly drain out with the water. The draining process, called leaching, was repeated many times, taking several hours to complete. After this process was complete, the leached acorn flour was placed into a basket that had been soaked in water. Basket fibers expand when wet, and the coils tighten so much that the basket becomes watertight; water does not seep through. The flour was then mixed with water. To cook the acorn soup, cooking rocks, basalt or soapstone, were heated in a fire (only these kinds of rock can withstand such heat, other types of rock might explode). Using wooden holders like tongs, they lifted the rocks, quickly dunked them in water to wash off the ashes, and then dropped them directly into the acorn soup in the basket. Within minutes, the soup would come to a boil and be ready to eat. Sometimes the acorn soup was poured into cold water; it would then congeal into something like a dumpling." -- from DeLaveaga Elementary School, Santa Cruz, CA