What makes sensible people do dumb things? We all know it happens.

The author of a famous text book on wills died at a ripe old age -- without making a will. Albert Einstein couldn't remember his own address.

Emily Dickinson didn't want strangers seeing her handwriting, so her sister addressed all her envelopes. Hans Christian Andersen was so worried about being buried alive, he posted a sign by his bed that stated "I am not really dead."

Perhaps these are personal quirks rather than stupidity. Still, it does seem that intellect is no guarantee against foolish behaviour.

But there's one kind of pitfall we all tumble into. Call it the Misleadingly Obvious trap.

At the roulette table, most of us believe that if black comes up several times in a row, white is more likely to follow. In fact, the odds are the same at each turn of the wheel, no matter what went before.

But while we may understand this on some intellectual level, confronted with a run of black numbers, we still plump for white. It just seems obvious.

Put another way, we let our instincts do the work our brain should be doing. Instead of stopping to think, we allow appearances to deceive us.

Think you're immune? Here are three puzzles designed to pull the wool over your eyes. See if you can avoid being taken in.

No. 1: You write Christmas cards to five friends, and address five envelopes. Then in a fit of absentmindedness, you stick the cards randomly in the envelopes. What are the odds that exactly four of the cards will go in the correct envelopes?

Hint: This looks like a math problem, but it's really a word trap. The solutions to all three puzzles are given on the opposite page.

No. 2: You have just tested positive for a disease that afflicts one person in 1,000. The test is 100 per cent accurate for people who have the disease and 95 per cent accurate for those who don't (this means that five per cent of people who do not have the disease will be wrongly diagnosed as having it). What's the probability that you actually have the disease?

Hint: Most physicians confronted with this puzzle guess the mid-point between 95 and 100. The actual probability is much lower.

No. 3: This teaser generated huge controversy when it aired on an American television talk show. People are still arguing over the solution because it appears so counterintuitive.

You're on Monty Hall's show, Let's Make a Deal. The host shows you three doors. He tells you that behind one is a new car. Behind each of the other two is a basket of lemons.

You opt for Door 1. The host, who knows what's behind each door, opens Door 3, revealing lemons.

Then he gives you a choice. You can stay with Door 1, or switch to Door 2. What should you do?

Hint: It might seem that it makes no difference which you choose -- the odds are the same either way, exactly 50-50. That's just how it looks.

If you aced all three, we would not want to face you in a poker game. One or two right, and the provincial government needs your help with its budget numbers.

But if you struck out on all three, well, not to worry. You're in good company. After the solution to the third puzzle was published, thousands wrote the talk show that aired it to protest.

Among those who disagreed were statisticians, mathematicians and hundreds of people with PhDs. Which just goes to show: Even the cleverest among us mess up.