Cookies are soft and normal when they are freshly baked, and will become brittle after cooling. If it is still soft after cooling, it means that the baking time is not enough and the water is not completely dry. The solution is to bake cookies in the oven for a few more minutes. If the biscuit is dark enough, but still soft after cooling, it may be because the oven temperature is too high. At this time, try to reduce the oven temperature and prolong the baking time of cookies. In addition, the bigger and thicker the dough, the harder the moisture is to dry, and the easier it is to bake soft cookies. On the contrary, the smaller and thinner the dough, the easier it is to bake crisp cookies. So, if you can't always bake satisfactory cookies, try making them smaller.