These are almost impossible to activate. The inner bag just will not break and yet the full length ones by the same manufacturer work really well - hold upright and squeeze the sides together. These small packs have a design fault. (And when you can get one to break, the cooling effect is very poor and short-lived compared with the full size ones. Don't waste your money on these. Buy the long ones.Bought these in January to have I. The house in case of emergencies. Only needed to use one today - after I had a fall. Opened the box carefully. Read instructions on the back of a pack. Tried one. Didn’t work. Tried a second one. Didn’t work. Tried a third. Saw. When it didn’t work either I decided they must all be faulty. LookedOn Amazon to return them but it’s too late. Can’t feel the inner sachet (as described on the pack) at all - you are meant to burst it (without opening the pack) and then shake it vigorously, to turn the pack cold. These are small and square so you cannot twist it or squeeze it much as it feels like it will pop open, and I couldn’t feel anything like an inner sachet (through the pack). AllI could feel were large granules. Squeezed and then all 3 of them vigorously as instructed, just hoping I’d managed to burst the elusive inner sachets. Nothing happened. So I’ve nothing to put on my swollen knee. Can’t even get a refund for them. I’d have contacted the seller but it’s too late as it is 3 months since I bought them. Would have given these no stars if that was an option.These are great to have on standby if you can't get to a frozen ice pack.I wanted small ones to use on my ankle/heel as I have achilles tendinitis. I read reviews saying they're difficult to pop the inner bag to activate and the first one I had was so easy. After that first one, I've struggled and feel like I'm going to pop the outer bag. With a bit of jiggling it does eventually activate but it is tricky.Instantly cold, needs shaking about to make it evenly spread. I tried wrapping in a towel and no cold comes through at all so I put it directly on my skin - which I know is not recommended use. I've not had any skin irritation at all.Lasts a good 20 minutes and fine for what I need it for.Have been using an ice pack that you place in the freezer for a pulled hamstring injury.As I was travelling with no access to a freezer I decided to purchase the Koolpak Ice Packs.They are very easy and convenient to use especially whilst travelling.The only issue for me is I didn't find them anywhere near as cold as the packs you place in the freezer.These are are good item but you might need to use more than one at a time to achieve the cold temperature required.Having suffered damaged my ankle ligaments from a bad tackle during football, these were a god send in recovery.Fortunately we had one of these in the first aid bag so could get cold compression on the ankle instantly.These things get really cold, so definitely follow the instructions to only use on and off for 10-15 mins etc.I subsequently bought this job lot of ice packs to ensure we can always have a few in the bag and they’ve already come in handy.