At Har Sinai we are told VECHOL HA’AM RO’IM ET HAKOLOT, “And all the people saw the thunder…”. (Ex. 20,16) Rashi’s comment is: they saw that which should be heard, something which is impossible to see on any other occasion. (Today we can see sound waves.)
Of course, this is a difficult concept for us to understand. Rabbi Moshe Feinstein expounds this text in an intriguing manner. Hashem wanted to teach an important lesson in the study of Torah. It is not enough to have a superficial knowledge of the Torah. A person should strive with all his intellect and all his abilities to understand the lessons the Torah imparts clearly – as if they were spelled out in front of his eyes.
Anyone can hear thunder. It takes a special ability to connect it with lightning and understand how one is a result of the other. The miraculous level of seeing thunder, meaning understanding what it is, represents a level we must strive to reach.
This is true about anyone who is learning any subject. He must strive to fully understand the subject matter. It is especially important for people responsible for instructing others. Parent should be sure and clearly know what they want to impart to their children. Teachers should have a clear understanding of the issues and the subject matter they wish to teach. One must clearly be sure of what he knows before he can transmit it to others.