Play of the Cards - Drawing the opponents' trumps

Contract : 4
Lead: King of Spades

			9 4
			K 9 7 2
			Q 10 5 2
			K 6 2
	K Q J 7 5	N W E S			10 8 6 3
	10 6 3				J 8
	A 6				9 8 4
	Q 10 7				J 8 5 4
			A 2
			A Q 5 4
			K J 7 3
			A 9 3

The bidding:

South: "I open";
North fills in his little paper and writes: (2, 4, 4, 3) and 8 H points.
South notes the eighth fit in Hearts and finds:
18H + 1D + 1D in North, so 28 DH points : that's enough to bid 4

Let them play the contract after checking the lead…..

There are those who will start by playing Diamonds and will be trumped. The concept of Trumps is not at all innate and you will frequently notice in this type of deal that certain players will still have all their Trumps with only five or even four cards left to play! Simply because they haven't the notion of the inherent danger of being trumped by the opponents and they have the vague belief that trumping is the sole right of the declarer.
It will be easy here to put your finger on their error by making them realise that if the opponents had not had any trumps they would not have trumped in. They will deduce for themselves the principle of the lesson: