"no no no ...do that other one again"
"Dude, I just showed you two illusions. Maybe you need to cool it a little, I don't want to overload you"
If they are your only audience then it's ok to put them on the spot a little. In which case:
From here they could say
Because it was totally awsome!
in which case;
"It's never so awsome the second time. That'd take the fun out"
Because I want to see how you did it!
" *smile* "
Treat those responses as flattery, it seems to be a very common reaction to a trick which is either really visual or really stumps them.
The answer to watch out for is:
I wasn't really paying attention
I didn't understand what was going on
Either they are lying, in which case you have failed to build a good relationship with your audience or maybe they just aren't the sort of person who will appreciate you, or, much worse, they really didn't understand what was going on. This is your fault, and you should think about how you can make your presentation clearer and more attractive. Either way, I think the best thing to do is apologise for being unclear. Either they deserve that apology, or it'll make them feel guilty about lying to someone who is only trying to entertain.