Us­ing id­ioms is a dou­ble-edged sword. One side lets you cut to the point. You say a phrase, and it's so load­ed of mean­ing, it's a short­cut to what you mean. One side can cut you, if you just use it and you use it wrong.

Let's con­sid­er Beat­riz Sar­lo, a re­spect­ed ar­gen­tine in­t­elec­tu­al. She writes op-ed col­umns in a news­pa­per. She wrote one to­day. It end­s:

To­tal, Boudou, sin bromi­ta al­gu­na, debe ade­cuarse a lo que le to­que, obe­de­cien­do el viejo re­frán de que a un ca­bal­lo re­gal­a­do no hay que ex­am­i­narlo para ver si viene com­ple­to.

Which I trans­late as:

In any case, Boudou, jokes aside, has to ac­cept what he get­s, fol­low­ingth­at old ad­vice about not ex­am­in­ing gift hors­es to see if they are whole.

One pe­cu­liar thing here is that we have al­most the ex­act same say­ing in span­ish and eng­lish. In eng­lish you don't look gift hors­es in the mouth. In span­ish you don't check the teeth of gift hors­es. The prob­lem here (if I may be so pedan­tic (yes you may (thanks oth­er me! (my plea­sure)))) is that Ms. Sar­lo has no freak­ing clue of what that mean­s.

Sup­pose you are buy­ing a horse. You would check the mouth be­cause you want to see if the horse is young or old (c­i­ta­tion need­ed? Here is google's first re­sult). That's im­por­tant when you buy a horse. It's, on the oth­er hand, re­al­ly use­less and rude when you are get­ting a horse as a gift:

Nice guy: Hey, here's a horse!

Rude mo­ron: (looks in the mouth) No thanks!

That's why you don look gift hors­es in the mouth. And that's al­so why you don't check ama­zon to see how much the book you got cost­s.

On the ther hand, if you were a Tamar­i­an, you could look at the horse's mouth and say "Tem­ba, at rest".