This is a classic lateral thinking puzzle. To solve it, we must break down the assumptions hidden within the phrase and explore unconventional interpretations.

Step 1: Deconstruct the Riddle's Components

• The Factory: A place of industrial production. We assume it makes a tangible product, but this is our first assumption to challenge.
• That Hid: An action of concealment. This implies a duality: what is seen is not what is real.
• Its Real Production Line: This is the core of the puzzle. The existence of a "real" line necessitates a "fake" or "decoy" line. The factory is actively hiding its primary function.

Step 2: Challenge the Core Assumptions

The most common assumption is that a "factory" produces a common good like cars, furniture, or electronics. Let's challenge this. What if the "product" is not a physical object? What if the "factory" itself is a front for its true purpose?

Step 3: Explore Alternative Interpretations

Let's consider types of "factories" that might have a reason to hide their primary function.

• Possibility A: A Factory as a Front. This is the spy movie trope. A seemingly innocent factory (e.g., a toy factory) hides a secret production line for illegal goods (counterfeit money, weapons, drugs). While plausible, it's a bit cliché and doesn't fully explain the elegance of the riddle.

• Possibility B: A Factory Producing an Abstract Product. What if the "product" is an idea, a person, or a system?

  • A school could be seen as a factory that "produces" educated individuals. Its "real production line" might be a hidden program for training spies.
  • A prison is a factory that processes human beings. Its visible production line might be the license plate workshop, but its "real" production line is the hidden, systematic process of producing conformity and rehabilitation.

• Possibility C: A Play on Words and Function. This is often where the most elegant solutions lie. Let's reconsider the term "production line" in a context where the primary product must be kept secure.

Step 4: The Solution - The Government Mint

The most fitting and classic answer is a government Mint.

1. The Factory: A Mint is, by definition, a factory. Its purpose is to produce currency.
2. The Production Lines: A modern Mint operates with at least two distinct production lines.
   • The Decoy/Fake Line: This line is often visible to the public. It produces commemorative coins, medals, and other numismatic items. Tours are conducted around this line to showcase the Mint's craftsmanship and engage the public. This is the visible production line.
   • The Real Production Line: This is the high-security, high-volume line that produces the nation's everyday currency—the coins and banknotes used by the public in daily commerce. This line is the economic engine of the country.
3. The "Hiding": The Mint hides its real production line for critical security reasons. Allowing the public to see the machines that print money would be a massive security risk. Therefore, the real, functional, and economically vital line is kept in a secure, non-public area.
4. Conclusion: The Mint perfectly fits the description. It is a factory that hides its real production line (for everyday money) behind a fake, public-facing production line (for collectibles). The "hiding" is not for nefarious reasons, but for the essential security of the nation's economy.

Final Answer

The factory that hid its real production line is a government mint.

The public is shown a visible production line that makes commemorative coins and medals, which serves as a decoy. The actual, high-security production line that creates the nation's everyday currency is kept hidden from view for security reasons. Thus, the factory hides its most important and real production line from the very public it serves.