Sōmen (素麺?) are very thin white Japanese noodles made of wheat flour, less than 1.3 mm in diameter. The noodles are usually served cold. The noodles' diameter is the chief distinction between sōmen and the thicker wheat noodles hiyamugi and Japanese wheat noodles udon. Sōmen noodles are stretched when made, as are some types of udon noodles. The dough is stretched with the help of vegetable oil to make very thin strips and then air dried.