Less/ fewer.
If the thing about which you're talking is finite - that is to say, countable - use "fewer". If it's infinite - that is, you can't count it - use "less".

Hence one beach may have less sand than another, but it doesn't have less grains of sand. It has fewer.

It won't do to point to the fact that Tesco has a "Ten items or less" checkout. Tesco is wrong.
( Enzyme is powered by sunlight, Thu 8 Apr 2010, 15:46, 7 replies)

My mnemonic
If you have 20 30 stone women and 2 go away you have fewer fat chicks, however if they each lose 5 stone you have less fat chicks.

Obama made a less/fewer error in a speech running up to the election, and immediately corrected himself, which probably secured a few grammarian voters.
(teebie jeebie, Thu 8 Apr 2010, 15:51, closed)

Haha, I like your mnemonic.

( Bob Todd whee, Thu 8 Apr 2010, 15:52, closed)

I love you
and I love Waitrose for having 'ten items or fewer' checkouts.

I was taught continuous/discrete rather than infinite/countable, which I think makes more sense: 'I spent less time than him in the queue' refers to a continuous variable (time), but it's not infinite. I didn't spend infinitely less time in the queue.
( Bob Todd whee, Thu 8 Apr 2010, 15:52, closed)

I was going to add Waitrose for getting it correct
Unlike Asda
( sandettie light vessel automatic New Twitter - @bollocksreally, Thu 8 Apr 2010, 16:04, closed)

Tesco
Also talk about "an unlimited amount of DVDs" on their DVD rental website, which is also wrong for the same reason.

Stuff: Use "less", "how much", "amount of"

Things: Use "fewer", "how many", "number of"
(Vice Admiral Sir Charles Cockbrush KCB MBE DSO, who is finding it increasingly difficult to log into b3ta.com wants to duck your aunt and kick your puppy, Thu 8 Apr 2010, 16:24, closed)

And it's just wrong anyway
because of course they can't actually have an unlimited number of DVDs.
( Bob Todd whee, Thu 8 Apr 2010, 17:05, closed)

TESCO
The sign actually says "Up to 10 items" if you want to be pedantic... Check it next time you're there.
( user_names_suck, Mon 12 Apr 2010, 23:27, closed)