The Sunday Times' tech correspondent Danny Fortson brings on Cal Henderson, co-founder and CTO of Slack, the wildly popular business messaging platform, on founding photo sharing company Flickr (2:30), its ill-fated sale to Yahoo (6:00), starting again (9:00), the accident that became Slack (10:30), being a unicorn (14:30), his early days in London (17:00), learning to be an optimist (19:15), why he's not worried by Microsoft (21:00), 'Calloween' (25:00), the power of emojis (28:00), what's it's like to be personally worth hundreds of millions of dollars (32:45) and loving Lego (34:00).

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