In plain words
Brahmacarya does not mean merely living without the dharma of the householder. Brahmacarya is to keep one's mind ever moving in the principle of Brahman.
Pūrṇam adaḥ pūrṇam idaṃ pūrṇāt pūrṇam udacyate pūrṇasya pūrṇam ādāya pūrṇam evāvaśiṣyate
That is full, this is full. From the full the full arises. When the full is taken from the full, the full alone remains. That is, Paramātma and Prakṛti are both full in their very form. So it is only when every work we do takes on a full form, only when it carries the principle of fullness, that we can experience this fullness and complete Ānanda.
For the thoughts to be strong and content, the body must be nourished
If we are to reach the Supreme Principle, which is fullness itself, all our actions must be in a state of fullness. Today we do not do even the smallest task with full faith; we do not carry it out with full sacrifice. We carry out every act incompletely. How can these incomplete acts reach the Paramātma, who is fullness itself? Only in a healthy body can a healthy mind stay. Only when mind and body are healthy can right resolves grow. When we want our thoughts to be sacred and spotless, the body must be strong and the mind content. So for the thoughts to be strong and content, our body must be nourished.
In Rome, a country famed for its civilisation since ancient times, those who built up the body most, who kept it most healthy, were thought great. Our health is our true wealth! For students, boys and girls, a strong body and contented thoughts are essential today.