Regarding dehydration (much less severe than heat stroke which befell the two brothers), if planning on riding in Morocco at this time of year it's worthwhile mentioning that there are two types of powders that deal with it.

• the relatively expensive Dioralyte/Rehydrat/[+Supermarket versions] which I'd classify as 'medicinal' for when you're losing it as Tim describes above. About 50p a shot (200ml). Also good if you have the runs and are losing water that way.

• the less expensive 'preventative' tablets like High5 Zero or Nuun tablets, or 'isotonic' tubs of powder like Gatorade sold for sports activities. Tabs about 60p/litre.

Dioralyte/Rehydrat especially are much more than just sugar and salt, though that mixed at 8:1 is better than nothing if you're feeling crook. They include a specific ratio of other key minerals to rebalance your body's electrolyte levels, the medium through which signals are transmitted back to the brain and why your senses (eyes, balance, touch, etc) all start to go when the electrolyte balance is off as you lose minerals in your 'salty' sweat. I've watched a person staggering around and seeing double come round in minutes after taking a Rehydrat.

I always carry a few Tesco-branded 'Dioralytes' on my travels but rarely use them these days unless I get the runs. I think that's partly because I now regularly use Zero tablets when doing energetic stuff in warm places.

And if i'm in a hot place (as we were in AZ/Utah a month ago when temps hit seasonal records), I'd buy it in powder form by the tub. We got through a kilo in ten days but had no headaches or other dehydration symptoms walking 5 hours at up to 40C.
On ebay High5 Isotonic Energy Drink 2kg (not quite the same as sugar-free Zero) goes for 23 quid and they say makes 24 litres @ up to 40°C, maybe twice that if it's less hot.
When I do my camel walks (Alg winter, <30C) I take one tube of H5s per four days, plus plenty of Rehydrats to dish out as needed.

Riding one time in southern Algeria in September (a big mistake...) I was having to stop and drink every 30 mins (10 litres a day and another couple at night trying to keep up). So that tub would have lasted me two days. I appeciated then how, without water or shade but not exerting yourself in any way (like the puncture example given above) you'd be too far gone to help yourself within just a few hours.

Just drinking water was not enough. After a few days I came over all groggy, realised what was happening, did the 8:1 thing and recovered.

btw, plain salt tablets are not the same, and are now discredited AFAIK. The one time I tried them after a long day on the sands in Mori with the runs, I threw up (normal reaction when ingesting too much salt at once). 8:1 is better.

So if you're riding in Mk in summertime I'd strongly recommend drinking something like Zeros regularly (along with taking all the other usual precautions to reduce water loss which others have described).

Ch

PS:
That did not look right to me so seeing as we're all having a go at Tim's stats, I'd suggest this may not be accurate.
A quick Google to a health site came up with:
4-5% = mild dehydration.
'Severe dehydration (>/= 7%)'
So 10% down and you're collapsing or worse.
I'd guess thirst kicks in at 2-3% - the first symptom of gradual dehydration that I experience. Which is another good thing with those tablets: the fruity flavours make you want to drink them more than plain water.