Perplex City - We Love Puzzles

Since all of the jars are labeled wrong the one that says Mixed has either all Sweet or all Regular. After you taste a pickle from it you will know which one, let's say it was Regular. So now you know the jar labeled Mixed has Regular pickles and you have 2 jars left one labeled Sweet and the other labeled Regular. The one labeled Sweet has to contain either Regular pickles or Mixed pickles (otherwise the label would be correct). It can't be Regular (since that was in the jar labeled Mixed) so it has to be Mixed. Thus, the last jar which is labeled Regular has to contain Sweet pickles, the ones you want. Alternatively, if the pickle you tasted was Sweet in the Mixed jar you would know right away that that was the jar with Sweet pickles in it.

So by tasting one pickle from the jar labeled Mixed pickles, you will know which jar contains the Sweet pickles.

To further emphasize this, imagine tasting ONE pickle from the Regular jar. If you tasted a Regular pickle you would know it was the Mixed jar, but if you tasted a Sweet pickle it could be either the Sweet jar OR the Mixed jar. There's no way to tell for sure without tasting more pickles and the same scenario applies to the Sweet jar.