Moses, the great lawgiver of the Israelites, goes up on a mountain to have a conversation with God. This conversation lasts for 40 days and 40 nights (without food or water - in the desert) and during this time God pounds out the ten commandments on some rocks. Those rocks were pretty hard so it's no wonder that it took 40 days and 40 nights to do it (even though God had created the whole universe in only 6 days).
Anyway, after shaping the rocks into tablets, Moses makes his way down the mountain. No doubt, he's pretty thirsty from doing without water for 40 days and 40 nights in a desert environment, but Moses is one tough prophet and he doesn't need water to live like the rest of us.
Meanwhile, Moses' #1 priest, Aaron, has made a golden calf and the people are having a high old time and partying up a storm when Moses arrives at the foot of the mountain. Wow, is Moses and God ever mad at what they see! These people had already broken one of the ten commandments - which they hadn't seen or even knew about yet - but, ignorance of the law is no excuse so a bunch of them had to die and the survivors of the wrath of God had to grind up the golden calf of pure gold and eat it.
So, the lesson here is that even if God doesn't have a law against whatever it is you want to do, he probably has one coming down the mountain right now. You see, then, that you have to be able to read God's mind and anticipate that whatever it is you enjoy in life - it's probably against one of God's laws that he hasn't mentioned to you yet. Ignorance is no excuse.