A shop sign reads: "Some of our sweets are sugar-free." Ishani concludes from this that…

Why

Ishani is using 'some' the way it often works in everyday speech, where saying 'some' hints that not all. But as a plain statement of fact, 'some of our sweets are sugar-free' only promises that AT LEAST ONE is; it does not rule out that ALL of them are. The shop could have made its whole tray sugar-free and still put up that sign without lying. Test: imagine the shopkeeper instead says 'some of these sweets contain sugar', and then you find that every single one does — was the shopkeeper lying? No, because 'some' was still satisfied. The same logic applies to the sugar-free sign. Her friend is right: 'some are' leaves open whether the rest are or are not — to know that some are NOT sugar-free you would need that stated separately, or you would have to check.