There are two sets of stumps with two bails at the top at either end of the pitch. They are all made of wood. Nowadays, one can read vertically the names of the sponsors on the stumps. This advertisement fetches them a lot of money as it can be seen on the television screen.
The sight of the stump sent cartwheeling by the bowler is very exciting, except for the poor batsman and his teammates! For the record, Ernie McCormick of Australia once sent a bail flying a distance of 67 yards!
Sometimes two of the stumps are flattened, but India's Madanlal had his off-stump and leg-stump completely uprooted while the middle stump surprisingly remained standing. This happened when he was bowled by Mike Hendrick in the Old Trafford Test in 1974.
Although the gap between adjacent stumps is sufficient enough to prevent the ball from passing through, a strange incident took place during a Test between Pakistan and South Africa in 1998. Mushtaq Ahmed of Pakistan 'bowled' Pat Symcox, but the ball passed through the stumps without the bails being dislodged! Symcox survived. But how did that happen? Probably, the stumps (wickets) would have been remade a little earlier after a run-out attempt. While remaking the wicket, one of the stumps (naturally the outer one) must have been slightly loose at the base and slightly tilted, which the umpire may not have noticed. The umpire should always ensure that the stumps are perfectly positioned when he 'remakes' them.
There have been instances in Test cricket wherein a bail has jumped into the air, but has lodged back into its original position. Neil Harvey of Australia and Hanif Mohammed of Pakistan were the beneficiaries. As per the law, mere disturbance of the bail from the groove does not constitute a broken wicket. It is necessary for the bail to get completely dislodged from its original position at the top of the stumps. Even if the bail gets lodged between the two stumps, it is regarded as a broken wicket.
In a Sheffield Shield match in the late 30's, it so happened that a batsman was bowled. The ball hit the middle stump and it tilted backwards, but the bails remained intact. 'Not out' was the decision. How did this happen? It was a brand new set of stumps and bails. In the heat, the varnish of the bails had melted and they had got stuck to each other, a little above the level of the middle stump! Of course, had the middle stump been completely uprooted, the batsman would have been declared out.
If, while in play, both bails get knocked out, the fielding side can replace one bail and effect a run-out or pull one stump out of the ground either by throwing or with the ball in hand. If all the three stumps are lying flat on the ground, then one stump can be replaced and uprooted again with the ball.
Sometimes, if it is windy and the bails are being blown off frequently, the umpires could dispense with the bails at both ends. In such circumstances, a stump need not be uprooted. If the ball merely kisses the stump, it is sufficient to regard as the wicket being down. It is for the umpire to observe and decide. However, the bails can be reintroduced once the umpires feel that conditions have improved.