"There is little oxygen here."
Chodan savours his statement for a moment and when I laugh breathlessly, he smiles. Two days in and he has been the fastest by far, doubling the pace set by Anna and I, until he was but a lone figure in the distance.
I take another swig and hear Anna approaching. We perform the customary exchange, moving around to each other's side pockets and pulling out snacks (almonds for the way up, cashews for the way down), before leaning our rucksacks against a rock to take the weight off our shoulders and hips.
As we rest, we take in our surroundings. The mountain we hope to summit, Stok Kangri, looms to the right, its eastern face a cruel patchwork of vertical rock and snow.
To the left, the Stok River, every bit as icy as the glacier it springs from, and base camp, where we plan to spend a few hours resting before attempting the summit that night.
It is a landscape of severe beauty and inhospitable wilderness, a combination that is as deeply alluring as it is dangerous, and one that commands one's immediate attention.
Far too soon, our moment's rest is curtailed by Chodan's enthusiastic announcement.
"Just over the ridge. Twenty, maybe 30 minutes. There is base camp. Challo!" he says, using the Hindu word for "let's go".
STARS AND SILENCE
It is day two of our three-day ascent of Stok Kangri.
We had started out of the northern Indian city of Leh, a former desert trading post organised haphazardly along the Indus, a broad, glacial river which allows trees of startling green to grow on the otherwise barren plateau.
Leh seems in a state of permanent disrepair, or perhaps repair, and it is often hard to tell what is being built and what is being demolished. Hardly anything in the city looks completely complete, only the mountains of the Stok Range are resolute in their solidity.
The city's skyline is dominated by these vast peaks, which extend up to the snow line and beyond. They do not look that high, but the mind is tricked by scale, for many of them rise well above 6,000m.
Stok Kangri is the highest, at 6,153m, and it draws both the eye and the mind, its immediacy suggesting attainability.
It is not uncommon for visitors to Leh to experience a sudden urge to climb Stok Kangri, having had no prior intention of climbing the peak.
This is the case with our own last-minute inclusion of the mountain on our itinerary and, after Anna's three-week research expedition on the geology of the Indian Himalayas, it seemed the perfect send-off before heading home.
And so, we arrive at base camp, breathless and determined.
Having spent time high in the mountains collecting data for Anna's PhD, we are both well prepared to tackle the mountain and are both acclimatised and fit.
We have planned three days for our summit attempt, with one extra "weather day" as contingency, in case of inclement weather. Most groups plan for four to seven days, depending on levels of acclimatisation.
Our night at base camp is spent beneath a freezing sky, cooled by a wind from the mountains that mercilessly picks off the insulating layer of clouds. In our tents, we fight the cold in our sleeping bags, dressed in the clothes we will climb in.
With few clouds, the sky is redeemed by stars. The Ursa Major constellation shines brightly over Stok Kangri and the southern sky is illuminated by the Milky Way, two hundred billion stars appearing as a crevasse upon the surface of the universe.
Otherwise, the camp is dark and silent, everyone seeking respite from the mountain's icy breath.
Our stomachs are full of dhal and rice - the food of the Himalayas - as nutritious as it is filling.
We should be resting for the climb, but sleep proves impossible. Instead, we talk in whispers and play Rummy by torchlight, waiting for midnight to come and our adventure to begin.
Midnight comes and Chodan's voice reverberates through the dark with hushed alacrity. "Anna. Oscar. We leave now."
Our party of three is joined by another, Anchuk, who takes over from Chodan as our guide to the summit.
Anchuk is a stoic man and says little, but his experience on the mountain is unquestionable: He had stopped counting several years before, after summit number 127.
He will lead us to the top of the mountain, helping us to navigate across the glacier, up the snowfield and onto the ridge leading to the summit.