text: It's basically a loophole in the law with a long tradition.  When Congress approves a bill and sends it to the President, he is allowed 10 days to either approve (sign) or reject (veto) it.  If he does neither then the bill becomes a law by default.  However, if Congress adjourns before those 10 days are up, then the President can do neither and the bill will not become law.  The rationale is that the legislature has to be in session to affirm that a bill would become a law without the President's signature if he did nothing; if they're not in session, then a bill that the President has not acted on cannot become law.