So, I bought the cheapest AAA thinner at Do Home.  The plastic sealing ring came off OK but I'll be damned if I can get the cap off.  What's the secret?  

 

I tried pressing my thumb down while turning which the icon appears to be saying - but it just turns and that's it.

 

 

take the largest screw-driver you have in the left hand positioned in the corner of the can and your trusty hammer in your right and apply a meaningful blow directly to the screw-driver,  if left handed just reverse the hand positions, unused thinners can be stored stored in a beer bottle or 2.

3 hours ago, bankruatsteve said:

I tried pressing my thumb down while turning which the icon appears to be saying - but it just turns and that's it.

 

The cap doesn’t have a screw thread. Pushing down expands the base. 
ignore the hammer suggestion but push down on the cap and use a flat screwdriver to help lifting the cap off. 
 

you can use beer bottles for storage but should have plastic caps for them

No turning, just push hard in the middle until you hear/feel a click.

 

Finally got a round toit.  Apparently my thumb is not going to be a Lethal weapon any time soon - so I tried giving it a bang with a screwdriver handle and the cap popped up.  It has a plastic compression thingy inside. and pops back into place.  

 

I've never seen anything like this before but there you go.

8 minutes ago, bankruatsteve said:

I've never seen anything like this before but there you go.

Designed to be child proof. I still remember trying to open a bottle of panadol about 40 years ago and could not budge it. My 4 year old told me you had to push down and turn at the same time, he opened it at first attempt.

3 hours ago, bankruatsteve said:

Finally got a round toit.  Apparently my thumb is not going to be a Lethal weapon any time soon - so I tried giving it a bang with a screwdriver handle and the cap popped up.  It has a plastic compression thingy inside. and pops back into place.  

 

I've never seen anything like this before but there you go.

 

We had the same cap when I was a kid. push down (as others said) in the center of cap to release and it should lift off.

 

Squeeze the perimeter together and it should click back on...

 

 

This one has an insert that expands the rim when pushed (to open).  To close, just press on the rim and it snaps shut.

 

 

21 hours ago, bankruatsteve said:

This one has an insert that expands the rim when pushed (to open).  To close, just press on the rim and it snaps shut.

 

 

 

 

The same cap we had on our thinner can 60 years ago, except that ours was all steel but for a paper gasket.