The BFF's were standing in for Vernon Kaye re-branding the show 'Lads FM'.
However, their bromance was tested when Nick Grimshaw admitted that he often confused his friend Harry's vocals with fellow One Direction members.
That's a bit awkward.
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In a bid to remedy this, Harry Styles conducted a wholly unscientific test.
Playing six clips from One Direction's album, Nick had to guess which boy was singing in each clip.
Unfortunately Nick failed miserably, unable to identify Harry's clip.
Later on in the show, Harry also answered listener's questions.
And one fan called in with a potentially dangerous query.
The listener asked Harry: "How long is…your longest toenail?"
A thrown Harry Styles eventually said it was probably 3mm before bursting into laughter over where he thought the question was initially going.
Get asked that a lot then Harry?
Harry will be appearing on tonight's X Factor live show alongside his One Direction band mates.
The band released their highly anticipated fourth single this week 'Live While We're Young' which charted at number one on Wednesday's midweek chart.