Birds + windows =? The BBC Quiz show The Unbelievable Truth reckons that more than 2 million birds die crashing into window panes every day in the US. Tim Harford finds this, well, unbelievable. Marcus du Sautoy explores the maths in Mozart's The Magic Flute; a student who uncovered a mistake in a famous economic paper, which has been used to make the case for austerity cuts, explains how he did it.