Wrong apostrophe's

Tesco seem to piss incorrect signs out of their arse on a daily basis. For a litany of their agrammaticism see this collection. (Perhaps that's why the majority of the public incorrectly call them "Tesco's" - the grammar gods are redressing the balance for all the missing apostrophes).

I just walked past the Carphone Warehouse which informed me "iPod's for sale". Unless they were just selling the one - a strange economic practice for a major retailer - that means someone wasn't listening in class. So, if the title of this post didn't raise an eyebrow, here's a reminder on how they work.

1. The possessive apostrophe denotes belonging, like in "Tesco's fruit" or, if the word ends in an 's' "Ross' hair" (it's ok to stick another 's' on if you like, but I don't, e.g. "Ross's".)

However, there are a few exceptions including,

It's and its

'Its' is possessive but lacks the apostrophe because 'it's' [it is] usurps it. For example, 'It's impossible its wings came off".

'Whose' and 'who's'

'Whose' is possessive but lacks the apostrophe because 'who's' [who is] usurps it. For example, 'Beardface, who's Head of Hirsute Neuroscience at Oxford University, has a PhD student whose thesis is going to be focusing solely on the neural correlates of moustache perception.'

2. The contracting apostrophe

This simply eats up letters (e.g., don't [do not]). You should never need to use this in academic writing. Therefore, if you find yourself writing "it's" and "who's" in essays, don't: either you'll have used the wrong possessive or you are making an unnecessary contraction.