The political world was shocked Monday morning to see Donald Trump had taken the lead over Hillary Clinton in national polls. In a CNN poll released Monday and conducted Friday through Sunday, Trump leads Clinton by 5 percentage points in a four-way matchup that includes two third party candidates. That’s a 10 point swing in Trump’s favor over Clinton from a CNN poll conducted July 13 through 16.

But here’s why everyone should take this poll and all other polls for the next three weeks with a grain of salt: historically, polling shifts wildly during and immediately after political conventions and then settles down two to three weeks afterward.