The dragons' water bowls are freezing overnight, and Hiccup needs a solution.
Hiccup: "Berk is a cold place most of the year, and the school is no different. What's worse is that our dragons' water bowls are freezing overnight. We need to find a way to lower the freezing temperature of water. Go see Heather by The Lab."
1- Meet Heather by The Lab
2- Collect sand from Stoick Bucket
Bucket: "All this sand was lying on the beach, almost as if no one wanted it. You can have this much!"
3- Collect salt from Gobber
Gobber: "I have tons of salt ready, in case I need to throw it over my shoulder for good luck."
4- Collect sugar from Phlegma
Phlegma: "Here's some sugar for you, 'sugar'!"
5- Give the sand to Fishlegs
Fishlegs: "This should be very interesting! I'm so excited to try it out and write down the results! Personally, I think that the sand will just sink to the bottom."
6- Give the salt to Astrid at the School
Astrid: "This will taste like ocean water to Stormfly. Still, the ocean rarely freezes, so this may work! Thanks! "
7- Give the sugar to Snotlout
Snotlout: "Sugar? Blah. That is too sweet. I like sour things, like a real manly Viking. I guess it's worth a shot if Hookfang's water doesn't freeze again."
Heather: "Now, we have to have an important thing everyone needs for scientific discovery: patience. We need to give it some time to see if it works!"
Hiccup: "Since it's so cold, I need to make fire to keep warm at night. I don't want to waste Toothless's time either. Can you go to the Wilderness and collect six logs for me?"
8- Chop wood for Hiccup's fire (6)
Hiccup: "That's great!
Bring those logs back to me so I can get this fire started."
9- Take the wood to Hiccup
Hiccup: "These have some potential. Thanks! You stay warm out there.
You should go see how the water turned out. Go talk to Astrid, Snotlout and Fishlegs."
10- Meet Fishlegs at the school
Fishlegs: "Meatlug's water was still frozen in the morning. She melted the sand and made some pretty cool fireballs, though. I'm so happy she got to show off her skills."
11- Meet Astrid at the school
Astrid: "Stormfly's water bowl didn't freeze. The salt did the trick! It lowered the freezing temperature of water. Stormfly didn't want to drink too much of the salty water, though."
12- Meet Snotlout
Snotlout: "Oh come on! You really didn't think that by sweetening the water with sugar that it would lower the freezing point, did you? Hookfang just got really hyper! He's spewing fire all over the place!"
Hiccup: "Well, at least Astrid is happy! Now we know that when we need to lower the freezing temperature of water, we just need to add salt to it. Thanks for your help!"
Note: as of March, 9th 2017 this quest hasn't been updated and it asks you to talk to Stoick. Even if it tries to send you to school to talk to some characters, they aren't there.