After Moshe and Aharon bring the first plague (of turning the waters of Egypt to blood) upon Par'oh and the Egyptians, we are told that the wizards of Egypt did so as well and Par'oh hardened his heart and did not listen to Moshe's pleas.
The question discussed among the various commentaries on this point is: where did they get water to magically turn to blood? All water was already blood turned so by Moshe and Aharon!! What did the Egyptian wizards do?
There is a wide array of answers proposed as solutions to the above problem. The answer I like is suggested by a few commentaries, most notably the Ohr Hachayim: We know that the Egyptians could attain water and not have it turn to blood. The way they could do this was by buying water from a Jew. Any such purchased water would remain water. The wizards purchased water from the Jews and then proceeded to turn that water to blood.
Look how ridiculous that is! Moshe turns all the water of Egypt to blood. In order to replicate the event, Par'oh greatest wizards have to first purchase clear water from a Jew and then turn it to blood. If they were such great wizards, they should have turned some blood back to water (and then turned it back to blood again)!! They had to purchase the water from a Jew and then turn it to blood. Yet Par'oh used that as a basis for being obstinate and not believing in Gods might and Moshe's request. Par'oh was graspign at straws in order to not believe Moshe. Ridiculous!
A person's stubbornness can be his downfall. If he can learn to look at something objectively and evaluate the situation, he will be able to deal with it in a rational manner. When a person lets his subjectivity get in the way he cannot look and deal with it rationally. He needs to step back and look at it again and see what makes sense.