One benefit is that if you don't have access to water close to your garden site you can pack your bike basket full of bottles of water with their original caps on. Then switch to this one when watering.
It's really simple.
You will need:
needle (or nail if you want bigger holes more water flow)
thimble (or hammer if you go with nail size holes)
Tips for other uses are appreciated in the comments section.
Step 1: Find a suitable cap
My suggestion: Find a cap. Yes, go out in the urban jungle and forage. You probably won't have to walk very far before you find a recyclable bottle thrown into a garbage can. Take the cap from it (take the bottle only if you need it, there are probably someone out there who need that recycle-fee better than you. If your country have such a recycling system that is).
You may want to clean the cap before you move on to the next step..
Step 2: Pierce the cap
Depending on what you'll use the sprinkler for you'll have to decide how many and how big holes you want. The fist cap I made had not so many (15) but big holes (small nail) and quite good water flow.This time I want a finer sprinkler for delicate plants. Therefore I'm using a sewing needle. You'll have to decide what you need.
Another thing is to angle the holes so the water won't just go straight down but have a nice kinda 'sprinkling' effect to it.. ;-)
Gently pierce the cap from the inside. Try to start from the center and work your way out in circles. That way it'll look nice and give an even spray. When you've done all the holes push the needle through them all from the outside to make them evenly round and the same size.
like that y'all!
Now. Go out and help the world grow up.