"You never can tell. Perhaps in some cellar of one of the downtown buildings, for all we know. Personally, I've got an idea. It may be foolish, but I've been turning it over in my head for a few days, and the more I think of it, the more reasonable it seems." "Spring it." "You remember the day we were at the old mill?" "I'll say I do! Those fellows wouldn't let me dry my clothes in the mill after I'd fished that precious kid out of the water." "But one of them offered us a reward, didn't he?" "Oh, well--you can't take a reward for that." "That isn't what I'm getting at. Do you remember how the other man grabbed the bills out of his hand and turned his back to us?" "Sure! He said he wanted to see if they were fives or ones. But it _was_ rather funny that he turned his back to us. I thought so at the time. Still, he offered the money to us again." "But was it the same money?" Joe was silent. The idea had not occurred to him before. "Do you mean," he said at last, "that perhaps the fellow changed the bills while he had his back turned?" "Exactly." "But why should he do that?" "Don't you see? Perhaps the first bills were counterfeit. Perhaps the man thought that if we took the counterfeit bills and later found out that they weren't good, we would remember where they came from and start an investigation. This is only a theory, remember; but perhaps the reason he took the bills from the man they called Dock was to change them for good bills, so that we would have no cause for suspicion." Joe nodded reflectively. "By gosh, Frank, there may be something to your idea, after all. Say! Perhaps that's where the counterfeiting plant is. Right in the old mill!" "That's just what I've been driving at. There's something fishy about the old mill, for all their story that they're making a patent kind of breakfast food. That may be true, of course, but still--" "They didn't look very much like scientists to me." "To me, either." "But how can we find out anything more about the place than we know already? They won't let any one inside the mill, and it's quite evident that they don't want any one around the place at all." "What made me suspicious," said Frank, "was the fact that Paul Blum seemed to be heading for the mouth of Willow River that afternoon he got away in the motorboat. I began to wonder later if he might have been intending to make his way up as far as the old mill. Perhaps he is connected with the gang." "It looks reasonable. But if we show our noses around there they'll just chase us away." "There's Lester." "Lester?" "The boy we saved from drowning. We have him on our side anyway, I think. If we haven't, he must be a very ungrateful beggar. I'd just like to ask him a few questions about this patent breakfast food yarn."