That is the biggest mystery of them all.
5 seconds sound clip from the Willy Wonka & the Chocolate Factory movie soundboard.
You can hear this line at 00:10:03 in the Blu-ray version of the movie.
Quote context
[...]
- The factory started working again, full blast.
- And more delicious candies were coming out than ever before.
- But the gates stayed locked, so that no one, not even Mr. Slugworth, could steal them.
- But, Grandpa, someone must be helping Mr. Wonka work the factory.
- Thousands must be helping him. But who? Who are they?
- That is the biggest mystery of them all.
- Charlie Bucket? Yes, mister Turkentine?
- I shall need an assistant. Come and give me a hand.
- But mixed together in the right way, as only I know how, what do you think it makes?
[...]