Might just hose down the walk instead, just as much fun and they won't know it was deliberate. Hm, just have to try both, see which works better.
That's usually how it works though. Best thing to do is ask him where he was on a specific date. Works for the Captain as well. Worst case he actually remembers and goes on about that for a while. Best case he'll have to go think about it and you've bought yourself an hour or two.