I encountered the following sentence while reading a book:

We soon discovered that many slips are possible between the cup of a promising idea and the lips of real-life applications, and that only a thorough understanding of these intermediate steps can turn a promising idea into something really useful and practical.

I can figure out the meaning from the context:

There might be many dangers between ideas and applications of ideas. So the idea alone is not enough.

But I cannot figure out what the 'lips of' means there.

I encountered the following sentence while reading a book:

We soon discovered that many slips are possible between the cup of a promising idea and the lips of real-life applications, and that only a thorough understanding of these intermediate steps can turn a promising idea into something really useful and practical.

I can figure out the meaning from the context:

There might be many dangers between ideas and applications of the ideas. So the idea alone is not enough.

But I cannot figure out what the 'lips of' means there.