Stand over the pot and watch. A rolled disc slides in, sinks a little, wobbles, and then swells like a small gold balloon while the oil crackles around it. When that happens, nothing about it looks difficult. When it does not, the disc just floats there, flat and oily, and no one in the kitchen can say why. The shells for pani puri, dahi puri and some papdi chaat are tiny pieces of physics made by hand, dozens at a time. Each must puff into a round, stay round while it dries, and stay crisp for days. Failures almost always trace back to one of five causes: the dough's firmness, its thickness, its rest, the oil temperature, and what happens during the first seconds in the pan. Understand each and the guesswork goes away.
Dough hydration and stiffness
Mix fine semolina with a little plain flour and knead until the dough feels almost stubborn, much firmer than roti dough. A fingertip should leave a dent that barely fills in. Slack dough slumps under the pin, drifts into thick, lopsided discs, and tends to refuse to puff. Firmness matters because it gives the dough a wall strong enough to hold while steam rises inside. A soft wall bulges in one place and caves in another. Pour the water in a little at a time as you knead and stop as soon as the dough holds together. You can always add another splash, but there is no easy way back from a dough that has gone soft.
Rolling thickness: the single biggest variable
Roll thinner than your instinct says, to about 1.5 to 2 millimetres, which is nearer to a playing card than to a roti. At that thickness the top and bottom of the disc part company quickly as steam gathers between them. Anything thicker needs more pressure and more time, and the surface often browns and stiffens before the puri has finished rising, so you pull out a half-flat shell. The disc also needs the same thickness everywhere. A thick spot resists, and the thin dough around it balloons in a crooked way. Use a clean, lightly floured counter, start at the centre and push outward with a few firm strokes. Many tentative passes tend to leave ridges.
Why resting the dough matters
Cover the kneaded dough with a damp cloth and walk away for 20 to 30 minutes. In that time the semolina soaks up the last of its water and the dough loses its tension, so it rolls out evenly and puffs more reliably. Dough rolled at once keeps shrinking back, and sometimes tears. It can also fry up faintly gritty, because the semolina never finished drinking. Cover the rolled discs too while you fry in batches. Edges left in the open dry out and crack, and a cracked edge lets steam leak away before the puri can swell.
Oil temperature: too cool and too hot both fail
Bring the oil to about 180°C (350°F). A scrap of dough dropped in should bob up and start bubbling within a second or two. Cool oil is the first trap: the dough sits and soaks, steam builds slowly, and the puri either lies flat or rises weakly, then comes out heavy and greasy instead of light and hollow. Hot oil is the second trap: the surface sets so fast that the layers never have time to separate. You get a quickly coloured puri, flat or only part-way up, with small blisters in place of one smooth dome. The window between the two is narrow, and the temperature drifts as batches go in and out, so spend a minute testing with a scrap of dough before every new round.
The pressing technique right after the dough hits the oil
Here is the vendor's small trick. As soon as the disc settles into the oil, touch it with the back of a slotted spoon for a second or two. Press lightly so it stays under the surface and does not get squashed. Both sides then heat together, steam builds evenly across the disc, and the shell swells into a clean round instead of favouring one side. When the puri starts to inflate by itself, usually within a few seconds, lift the spoon away. Keep pressing and you push the air back out. This is a move your hand learns faster than your eyes. After a handful of tries you will feel how much pressure to use and when to let go.
Troubleshooting specific failures
A puri that lies perfectly flat was probably rolled too thick, dropped into oil that was too cool, or made from dough too soft to build a wall. One that splits open without puffing usually had a dried, under-rested edge or a thin weak spot that tore when steam pushed on it. If a shell swells beautifully and then sags as it cools, it left the oil too soon. It rose on steam alone, its walls never dried into something rigid, and as the steam inside cooled and shrank, the shell folded in. When a puri puffs and looks perfect but eats chewy, it is nearly always under-fried, taken out before enough moisture left the dough, or fried in oil so hot that the outside coloured long before the inside dried. For both of those, keep the shell in the oil a little longer at a steady, properly hot temperature, until it has had time to crisp right through.
Cooling and storing for lasting crispness
Let the fried shells cool entirely, uncovered, before they go anywhere near a lid. Closed up warm, they trap steam and go soft from the inside. A wire rack helps, with the shells in a single layer so air circulates around each one and the last moisture can escape. When they are cold, an airtight container keeps them crisp at room temperature for up to a week, since a properly fried shell holds almost no water and has nothing for mould to feed on. In humid weather they may soften anyway. A few minutes in a low oven brings the snap back.
Small habits that raise your success rate
Cooks who turn out perfect shells tend to share a handful of habits. They roll only as many discs as the pot can take at one time, so none sits long on the counter and dries. They keep a bowl of extra semolina or flour nearby for dusting, using as little as possible, because excess flour burns in the oil and clouds it. They fry a test puri first and judge the whole batch by that one. They remove failed shells instead of crowding the pot with them. And they write down what changed whenever a batch goes wrong, whether that was the brand of semolina, the weather or the rest time. Over a few weeks that record turns guesswork into a routine you can repeat.