This is a waterproof and dust/sand proof survival container/ On the threads it has a little but of ridges that makes the closing really strong it may look not so smooth but it makes it stronger and more tight. 

to make it watertight you just need a rubber band and place it in the slot. NO TPU REQUIRED

you have to turn the cap the left way to  close it

BUT WATCH OUT IF YOU DONT PLACE THE RUBBER BAND CORRECTLY IT WON,T BE WATERPROOF

And sometimes you need more rubber bands