Sometimes in a sketch I need to delete a bunch of lines, arcs etc. all at once.  So I drag a box around them to select them all, but usually Solidworks then assumes I want to add relations between them so the delete key doesn't work.  This has always been a problem, but in SW 2010 it seems to be worse.  The odd thing is, sometimes I can delete all the entities I've selected.  I'm mystified as to what the algorithm is, such that it sometimes works and sometimes doesn't.

How can I select multiple sketch entities and then delete them all with one keystroke?