One of the most difficult concepts to explain in Christian theology is the doctrine of the trinity. This doctrine is often used by Jewish and Muslim people to demonstrate how Christians are actually polytheists, not monotheists. It is difficult to explain to anyone how God can actually be three persons, and at the same time one person. Anyone who says differently is kidding themselves.
One metaphor that I have never heard used before to explain the trinity, but which I think could be really helpful, is the idea that God can be likened to H2O, or for that matter, any chemical formula that can be a solid, liquid, and a gas. H2O is always chemically the same, but yet it takes on three different, very distinct forms. Ice, water, and steam are all H2O, yet steam has been used to power engines, water makes for beautiful scenery, and ice is a nice complement to a drink. But the chemical formula is always the same.
This is the same with God in the Trinity. All three persons of the Trinity are of the same essence, yet they take on different forms. There is the God in heaven, God in the form of Jesus, and the ever ephemeral Holy Spirit. All three forms are essentially the same, yet they come in different forms.
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