What causes a engine to smoke?

I have a 94 BMW 525i and I just recently had that problem. My BMW ran hot, smoked and had water dripping coming from the tailpipe. First thing I thought was, a blown head gasket because the smoke was white. When I took the oil cap off, and there was this chocolate milk looking substance on the cap. I knew then that I blew a head gasket. Check your oil cap for this substance and if it's present, go to any auto parts store and buy a bottle of Prestone or Peak Super Radiator Flush, 2 bottles of full strength anti-freeze and 1 bottle of Blue Devil Head Gasket Sealer. It should come to around $110.00 for all. FOLLOW THE DIRECTIONS ON THE FLUSH BOTTLE AND THE BLUE DEVIL BOTTLE EXACTLY. NO SHORT CUTS. It worked for me and my BMW hasn't ran hot or smoked anymore. And the chocolate milk substance is gone as well. It took two days of driving the car before it COMPLETELY cleared up so, if you still see a little smoke and smell something strange, it will go away after a couple runs on the interstate.