static function getUnits()
    {
        return [
            static::PIECE,
            // Length
            static::METER,
            static::CENTIMETER,
            static::MILLIMETER,
            static::INCH,
            static::FOOT,
            // Weight
            static::KILOGRAM,
            static::GRAM,
            // Volume
            static::CUBIC_METER,
            static::LITER,
            static::MILLILITER,
            // Duration
            static::DAY,
            static::HOUR,
            static::MINUTE,
            static::SECOND,
        ];
    }