Much of the best food in the world is free. These are some of the foods which you can pick for free in Europe. Every autumn,people all over Europe go out into the woods and fields to look for mushrooms( 蘑菇). People in France, Germany,Poland and Italy are very interested in mushroom picking. You have to be very careful when you pick wild mushrooms. Some are delicious, but others that look very like them can give you a stomachache. Some kinds of mushroom are erous and can make you ill or even kill you. The most famous and delicious kind of mushroom is the truffle(块菌). It grows underground in forests, especially in Italy and France. It is very difficult to find. Farmers usually go out with dogs or pigs. When the animals get excited and start to dig, they know that there are truffles underground. They dig them up very carefully. Truffles are very valuable. They can be black or light brown or white. In the countryside, there are lots of different fruits to pick. In June you may find wild strawberries. The best time to look for them is in the evening. If you walk around in woods, sometimes you will smell them before you see them. The blackberry(黑莓) is also well liked. It grows wild round the edges of fields in September and early October. Blackberries are black and small and very sweet. You can eat them as they are with sugar and cream. Or you can cook them for four minutes with some sugar and a little lemon juice. You can also make a very good jam with them. What is the passage mainly about?