Conversation so far:
What was the Mirror challenge for Harry Houdini?
In 1904, the London Daily Mirror newspaper challenged Harry Houdini to escape from special handcuffs that it claimed had taken Nathaniel Hart, a locksmith from Birmingham, five years to make.
Who challenged Harry Houdini to escape from special handcuffs in 1904?
The London Daily Mirror newspaper challenged Harry Houdini to escape from special handcuffs that it claimed had taken Nathaniel Hart, a locksmith from Birmingham, five years to make.
Who was Nathaniel Hart?
Nathaniel Hart was a locksmith from Birmingham.

Question: What was special about the handcuffs?
Self-contained rewrite: What was special about the handcuffs made by Nathaniel Hart?