If you've solved a challenge (or if you made the challenge) and someone else can't figure it out, this would be a good place to help.
Clam Spammer, I can't figure out the Tricky 28 challenge. What is it?
Good to see there's still some interest in these challenges. You're not the only one to ask me about this particular challenge...
But I can't just tell you how to do it. For one thing, it would defeat the purpose, but also some people have used the same trick in their own level designs, so giving this away would also reveal the solution to other people's levels. Instead, I'll refer you (again) to the old challenge thread. (http://camanis.net/lemmings/lemmingswelt/forums/?topic=1117597280) Look for ccexplore's posts on 15 September 2005 regarding a 1-builder solution to Mayhem 17 ("Stepping Stones").
I will say that, while the trick itself usually requires a fair bit of work to set up (and in fact for this challenge it is more complicated in some ways), one major step of the process has been done for you by the level designers.
Oh, come on!
I don't want to give hints about someone else's levels. They might not appreciate it :)
There are replays of some of the old challenge solutions on the Lemmings File Portal - you might be able to pick up some ideas from those.