When you come across beneficial knowledge, it’s good practice to write it down as it helps you to remember the knowledge and allows you to later on review what you’ve learned. As Allah mentions in the Qur’an:
Read, and your Lord is the most generous, who taught by the pen, taught man what he did not know.
Suratul Alaq Verses 3-5
This is why it’s recommended for a student of knowledge to have a pen and a small notebook wherever he goes.