More than 200,000 fans gathered in Mexico City's historic central square, the Zócalo, last Thursday night to see Paul McCartney in concert.

In 1967, American-born animator Bob Balser was working on a project in Spain when he received an invitation to fly to London and try his hand at designing animated characters based on the Beatles. The project? A full-length cartoon feature film called Yellow Submarine. “They had to get started quickly, because they had a screening date set for July of the following year,” Balser recalls. “They had just one year to do an entire feature-length animated film.”

In The Beatles’ catalog, “Hey Bulldog” is a bridge between the psychedelic excesses of 1967 and the rock and roll revivalism they would pursue on the White Album and Let It Be. Written by John Lennon, the song is a straightahead rocker featuring a seductive boogie-style riff and some excellent aggressive lead guitar work.