The Three Keys

"Engrave on it with signet-ring [type] engraving: "Holy to Hashem"
  פִּתּוּחֵי חֹתָם קֹדֶשׁ לַי־הֹוָ־ה 
[Tetzaveh 28:36]

In Maseches Ta'anis (2a) R' Yochanan states that there are three maftechos, three keys, in Hashem's possession that He never entrusts to others:

• the key to childbirth
• the key to rain
• and the key to resuscitating the dead

The Gemara derived this from three verses:

Hashem does not entrust the key to childbirth to a messenger, as the verse states: "G-d remembered Rachel; G-d hearkened to her and He opened (vayiftach) her womb" [Bereishis 30:22]

Hashem does not entrust the key of rain to a messenger, as the verse states: "Hashem shall open for you His storehouse of goodness, the heavens, to provide rain for your Land in its time" [Devarim 28:12]

And Hashem does not entrust the key of resuscitating the dead to a messenger, as the verse states: "Then you will know that I am Hashem, when I open your graves" [Yechezkel 37:13]

These three keys, remarked the Vilna Gaon, are alluded to in the verse: "Pituchei chosam kodesh laHashem" - פִּתּוּחֵי חֹתָם קֹדֶשׁ לַי־הֹוָ־ה -

The acronym of the word  חֹתָם [Ches, taf, mem] which means "seal" hints to the following words:

Ches - Chayah [a woman who has recently given birth]

Taf - Techiyas HaMeisim [the resuscitation of the dead]

Mem - Matar [rain]

The verse can therefore be read as follows:  "The keys of [pituchei] childbirth, resuscitation of the dead, and rain - ChoTaM - are designated for Hashem's use only ["kodesh laHashem"].

Source: Rabbi Yisrael Bronstein