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The churning of the milk ocean first produced a deadly poison " halahala " which Lord Shiva swallowed. Only he could swallow it without being affected by it. However, Parvati pressed his neck so that the poison does not reach his stomach. Thus, it stayed in his throat neither going up, nor down. The poison was so potent that it changed the colour of Shiva's neck to blue. For this, Shiva is also called Neelakantha (the blue-necked one). Neela means blue and Kantha means neck or throat.