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You may have heard of something called genetic engineering. The idea is to take useful genes from some plant or animal and add the genes to other species. For example, it is possible to take a gene for human growth hormone and put that gene into bacteria. Then the bacteria start producing human growth hormone. Or you can take a gene that makes jelly fish glow and put it into a pig. Now you have glowing pigs. Scientists have really done these things.

Everyone knows that cows make milk. The question is, could you add a gene from a cocoa tree to a cow and have the cow make chocolate milk?

It’s not so farfetched an idea. For example, scientists have taken a gene from a spider and added it to goats. The new gene lets the goats produce spider silk proteins in their milk. Scientists did this hoping that they could filter the spider proteins out of the milk and make super-strong string from it. So why not add chocolate genes to a cow and get chocolate milk? Making a chocolate milk cow is not quite so simple. One reason is that chocolate milk does not contain just one thing. To make chocolate milk, you need to add sugar to the milk as well as cocoa. So scientists would first have to figure out how to make milk sweeter. Without the extra sweetness, the chocolate milk would not taste very good.

There’s also the fact that not just one molecule makes chocolate taste chocolaty. Several chemicals work together to create the taste of chocolate. So scientists would have to add genes for all these different chemicals to the cow, and get them produced in the right amounts.

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Finally, some chemicals in nature are not produced directly. They are created by little multi-stage chemical machines. Each stage of the chemical machine might have a gene that controls it. So scientists would have to add genes for each stage of the machine, and then get the machine to work properly.

Let's say that scientists could find answers to all of these challenges. Then, in theory, they could make a chocolate milk cow. But then there are some other problems. Some people like their chocolate milk to be really chocolaty. Other people, well not so much. Some like their chocolate milk to be really, really sweet. Other people do not. Some people like little floating marshmallows in their chocolate milk, other people don’t. Getting a chocolate milk cow to produce little floating marshmallows is really, really unlikely.

All of these differences mean that, in the case of chocolate milk, it is probably better to let people add their own sugar and chocolate to the milk to make their chocolate milk exactly the way they want it. For this reason, the chocolate milk cow will probably never happen.