At a point when Drona, as the Kuru commander is killing vast numbers of Pandava troops, Krishna advises Yudhisthira to adopt a plan to kill Drona.. As it is known that as long as Drona has raised his weapons he is invincible to all other warriors. So they plan to make Drona fall into grief so that he will drop his weapons at least temporarily, and to achieve this It was devised that it would be a lie about Aswathama the son of Drona.
Yudishtra's adherence to dharma does not allow him to say this. Krishna justifies this lie to Yudhisthira as necessary to the victory of morality in the war. As Yudhisthira continues to hesitate, his brother Bhima kills a known elephant in the Kuru legions called Aswathama and celebrates shouting "Aswathama is dead! Aswathama is dead!".
Shocked when the news reaches him, Drona seeks out Yudhisthira to ascertain the news, knowing that the son of Dharma would never speak a lie. Yudhisthira tells him that Ashwathama is dead, but mutters "the elephant...." ( Aswathama Hatha.(Aswathama is dead).. Kunjaraha (elephant))in an inaudible voice to prevent telling a whole lie.
Now convinced, Drona lays down his arms and sits in meditation. It is actually said in the epic that Drona's soul has already left his body through his mediation, but Dhristadyumna takes this opportunity, swings onto Drona's chariot, and chops off his head.
So my take on this episode....those of you who know mahabaratha....should be aware that Drona's son is chiranjeevi, the one who has no death, but even then Drona believed Yudhistra.It is because the circumstances looked convincing. the warfield, with many people killed and his belief on son of dharma, made him forget the very thing that his son had no death.
In today's world, seeing is also not believing. Be convinced of what is the truth, before you act. Circumstances and Environment can make you victim of lot of things. So try to isolate yourself from the situation and get a perspective. Though difficult will help you avoid lot of unwanted scenes and problems...
Though Yudhishtra had not technically spoken a lie, but his convincing drona was equal to an lie. That is why the chariot of yudhistra which use to always float in the air and never touch the ground falls to the ground after this lie.