When Christ was pierced with the spear, why did water come out?

On the cross, when the Romans pierced Christ with the spear, the scriptures say that water poured out: > But one of the soldiers with a spear pierced his side, and forthwith > came there out blood and **water**. (John 19:34) The Greek word used is ὕδωρ (hydōr) so it's unlikely that they meant, "fluids" or anything else, it's apparent that it was literally *water* that came out. Why did water come out of Christ when he was speared?