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Can't remember your phone number? Defeated by a tax and tip calculation? Simple Sudoku got you scratching your head?
Don't worry - we all feel a little stupid sometimes. Maybe you haven't been getting enough sleep. Maybe you had one too many glasses of cabernet last night. Maybe the sudoku was just extra hard today.
Then again, maybe all that crap you eat is addling your brain.
It's long been thought that nutrition can influence your mental faculties. The idea has given rise to hundreds of popular books and articles, and it's undeniable that energy deprivation or vitamin and mineral deficiencies can screw us up - if we don't eat certain things we need it can impair our physical and mental functioning.
However, more exciting to an obesity researcher like me is evidence that things we do eat - and often to excess - may also influence how we think.
For example, rats who were fed a high-fat diet for just ten days in a recent study not only got more exhausted when exercising on a treadmill - but also more forgetful and less successful at hunting down rewards in a standard maze puzzle.
And it gets worse. Not only might junk food make you a dim-wit, but if you eat enough of it to become obese it could permanently affect your brain.
In a study of elderly obese and lean humans, the obese ones showed greater shrinkage in frontal, temporal and subcortical areas, compared with their leaner counterparts. Some of this may have been due to the obese people having diabetes - which is already known to affect the brain - but the atrophy was still linked to weight even after factoring that out.
Sadly, the subjects in this study didn't do any tests or sudokus, so we don't know whether those with a shriveled hippocampus were any more dense than those without. But it's probably fair to say that - when it comes to brains - bigger is generally better.
All pretty scary, huh? Probably best to stick to salads and keep your weight down?
Not necessarily. Scoffing junk and getting fat might be mental murder, but nibbling lettuce leaves in an effort to slim down could make you feel even more foolish.
Anorexic patients perform worse at tasks designed to test ‘set-shifting,' a type of flexible thinking involving the prefrontal cortex of the brain (although this could well be a cause rather than a result of the disorder).
And women who say they're dieting to lose weight show slower reaction times, and poorer recall in word memory tests. This might be because they lack dietary fuel to get their brains going, but it's also because thinking about all the food you're not allowing yourself to eat is thoroughly distracting and a drain on your mental energy.
As with most other things in life, the wealth of evidence on the diet-cleverness problem seems to suggest that balance is best.
In fact, achieving optimum body weight and brain function is really very simple, if you think about it: Don't starve yourself so much that fantasies of french fries overwhelm your powers of reasoning - but don't stuff yourself so much that your mind goes into meltdown.
(And if you believed what I just said about weight control being easy, then you really must be stupid...)
