When Viswamitra did penance, Indra came in disguise and met the sage. He asked Viswamitra to serve him food. Viswamitra agreed. Indra said he wanted the food kept hot, while he was out having his bath. But Indra had no intention of coming back.
He stayed away for years. How could Viswamitra keep the food from spoiling, leave alone hot, for so many years? Viswamitra knew the power of the Gayatri mantra. So he put the pot of food on is head and recited the mantra repeatedly.
That was enough to keep it hot. Indra then came to him and certified that Viswamitra was indeed a great sage.
A similar incident is seen in the Ramayana, in the case of Sabari. Sabari kept fruits ready for Rama. She did not know when the Lord would visit. So she had been setting aside the choicest of fruits for the Lord for many years. But the fruits retained their freshness over the years, despite being kept for so long. Her love for the Lord made the fruits remain fresh.
In the forest, Rama visited the hermitages of many sages and was served feasts by all of them. When asked later whose feast he liked the best, He replied that he liked Sabari's serving of fruits the best. The reason was that while others had wanted Rama to bless their penance, Sabari had been selfless in offering Him fruits.
She expected nothing from Him, not even moksha. She knew that her acharya bhakti would see her through to moksha. So she confidently asked Rama to be a witness to her ascent to Vaikuntha, where she was going with the grace of her acharya Matanga rishi. Thus, Rama was witness to Sabari's ascent to Vaikuntha, said V.S. Karunakarachariar.
Sabari had served Matanga rishi with devotion. And she had been waiting patiently for Rama to come to the ashram. Her reward for service to the acharya was moksha.
In the Sri Vaishnava tradition, when a person departs from this world, he is said to have reached the feet of his acharya, showing that unselfish service to the acharya leads to moksha. Sabari had the rare distinction of the Lord Himself watching her departure to Vaikuntha.