Probably one of the first science experiments that we all did when we were little was pouring oil into a glass of water. And what do we learn? The two substances have different densities, and so it’s very difficult to completely mix them. Well, during Mass, we do a little “science experiment” as well, by mixing water and wine. Why, you say? Well, I’m glad you asked!
As the priest pours a little drop of water in the wine during the preparation of the gifts, he says in a low voice (a secret one!), “By the mystery of this water in wine, may we come to share in the divinity of Christ, who humbled himself to share in our humanity.”
This co-mingling of water and wine is a very ancient practice. It was part of the culture at the time of Jesus, and so it has always been assumed that he did the same at the last supper, tempering or diluting his wine slightly with water. So we see it referenced in Proverbs 9:5, and St. Justin Martyr (150 AD) and St. Cyprian (250 AD) both make reference to this in the liturgy very early on!
So what does it mean? Well, consider for a second that water is a pretty common thing. It’s used to water plants, it is very cheap at the store, and you even see athletes drink some, only to spit it right out on the field! Wine on the other hand is expensive, takes a long time to make, and you never want to waste it, because it is valuable.
So when we mix the two, it’s a symbol of what’s happening with the Eucharist. Human beings are amazing creations, but we’re nothing on our own – we need God. Unlike water and oil, water and wine mix completely, so if we are the water, Jesus is the wine, and we see the point of his becoming a man. St. Athanasius (296-373 AD) said, “God became man, so that man might become like God.” Jesus, the Son of God, (symbolized in this case by wine) completely humbled himself, so that we (symbolized by the water) could be built up and share completely and inseparably in his divine life.
In fact, that’s what happens at the Eucharist. When we receive the Eucharist, we are receiving Jesus’ Body, Blood, Soul, and Divinity, and we are joined in such a close union with God, a communion (see what I did there?) that our common human nature is mixed with the divine! Awesome, am I right?
Aedificatio Dei 
This is awesome. During daily mass i like to recite the silent words of the priest. I find that it enhances my prayer. But I didn’t realize the significance of the commingling. What is the significance of the water and blood that flowed from the side of Jesus when he was crucified.?
Seems to me (not a Catholic) that water and blood signify birth, since they are both present at birth. I believe that for Christ, it was the birth of the glorified body, the promise of resurrection, the hope of Glory!