When Rivkah promises to provide water for the camels of Avraham's servant after giving him a drink, she says "גם לגמליך אשאב, also for your camels I will draw water" (בראשית כ"ד:י"ט). The Abarbanel points out that Rivkah thought and was careful to say that she would draw water for the camels, rather than say that she would give the camels to drink. Rivkah was ever so careful not to say anything that would be untrue. She therefore said that she would draw water for them, but she didn't know if they would drink or not.
Rabbi Shmuel Walkin Adds to this explanation of the Abarbanel, saying that we see here how careful one should be to keep away from saying anything untrue. He brings an example of this from the practice of Rabbi Rafael of Bershid who was always careful not to say anything untrue. When it was raining outside and he came into his house and was asked if it was still raining, he would reply that when he was outside it was raining. He didn't want to say explicitly that it was raining just then because maybe after he got in the rain had stopped.
At first glance, this might seem to be very minor. After all no great harm could come from a misunderstanding on this subject. However , if one is careful to tell the truth regarding little and insignificant things, then he will surely be careful in more important matters. On the other hand, if one is careless with the truth on small matters, he may be tempted to lie in major areas as well. It is thus worthwhile for one to develop the habit of never telling a lie even regarding the smallest of matters. Once a person makes truth a part of his speaking pattern, as Rivkah apparently did one can be assured of representing the truth in all situations in life.