1 Arjuna said: What is that Eternal, what Self-knowledge, what Action, O Purushottama? And what is declared to be the knowledge of the Elements, what is called the knowledge of the Shining Ones?
In whatsoever wombs mortals are produced ... the great Eternal is their womb, I their generating father. 14:4
8:1-12
1 Arjuna said: What is that Eternal, what Self-knowledge, what Action, O Purushottama? And what is declared to be the knowledge of the Elements, what is called the knowledge of the Shining Ones?
2What is the knowledge of Sacrifice in this body, and how, O, Madhusûdana? And how, at the time of forthgoing art Thou known by the Self-controlled?
3The Blessed Lord said: The indestructible, the supreme is the Eternal; His essential nature is called Self-knowledge; the emanation that causes the birth of beings is named Action;
Krishna said,
4Knowledge of the Elements concerns My perishable nature, and knowledge of the Shining Ones concerns the life-giving energy; the knowledge of Sacrifice tells of Me, as wearing the body, O best of living beings.
5And he who, casting off the body, goeth forth thinking upon Me only at the time of the end, he entereth into My being: there is no doubt of that.
If you think about me at the time of your death, you'll enter into my being.
6Whosoever at the end abandoneth the body, thinking upon any being, to that being only he goeth, O Kaunteya, ever to that conformed in nature.
Whatever a person is thinking about at the time of death, that is where the soul of that person goes after death. [1]
7Therefore at all times think upon Me only and fight. With mind and Reason set on Me, without doubt thou shalt come to Me.
Think about me and only about me all the time, without doubt.
And fight.
8With the mind not wandering after aught else, harmonised by continual practice, constantly meditating, O Pârtha, one goeth to the Spirit supreme, divine.
9He who thinketh upon the Ancient, the Omniscient, the All-Ruler, minuter than the minute, the supporter of all, of form unimaginable, refulgent as the sun beyond the darkness,
10In the time of forthgoing, with unshaken mind, fixed in devotion, by the power of yoga drawing together his life-breath in the centre of the two eyebrows, he goeth to this Spirit, supreme, divine.
11That which is declared indestructible by the Veda-knowers, that which the controlled and passion-free enter, that desiring which Brahmacharya is performed, that path I will declare to thee with brevity.
12All the gates closed, the mind confined in the heart, the life-breath fixed in his own head, concentrated by yoga,
8:13-28
13"Aum!" the one syllabled Eternal, reciting, thinking upon Me, he who goeth forth, abandoning the body, he goeth on the highest path.
Those who recite "Aum!" while thinking about me are on the highest path.
14He who constantly thinketh upon Me, not thinking ever of another, of him I am easily reached, O Pârtha, of this ever-harmonised Yogî.
They who think about me, and never about anyone else, are harmonized Yogîs.
15Having come to Me, these Mahâtmâs come not again to birth, the place of pain, non-eternal; they have gone to the highest bliss.
16The worlds, beginning with the world of Brahmâ, they come and go, O Arjuna; but he who cometh unto Me, O Kaunteya, he knoweth birth no more.
17The people who know the day of Brahmâ, a thousand ages in duration, and the night a thousand ages in ending, they know day and night.
18From the unmanifested all the manifested stream forth at the coming of day; at the coming of night they dissolve, even in That called the unmanifested.
19This multitude of beings, going forth repeatedly, is dissolved at the coming of night: by ordination, O Pârtha, it streams forth at the coming of day.
20Therefore verily there existeth, higher than that unmanifested, another unmanifested, eternal, which in the destroying of all beings, is not destroyed.
21That unmanifested, "the Indestructible," It is called; It is named the highest Path. They who reach It return not. That is My supreme abode.
22He, the highest Spirit, O Pârtha, may be reached by unswerving devotion to Him alone, in whom all beings abide, by whom all This is pervaded.
23That time wherein going forth Yogîs return not, and also that wherein going forth they return, that time shall I declare to thee, O prince of the Bhâratas.
24Fire, light, day-time, the bright fortnight, the six months of the northern path—then, going forth, the men who know the Eternal go to the Eternal.
25Smoke, night-time, the dark fortnight also, the six months of the southern path—then the Yogî, obtaining the moonlight, returneth.
26Light and darkness, these are thought to be the world's everlasting paths; by the one he goeth who returneth not, by the other he who returneth again.
27Knowing these paths, O Pârtha, the Yogi is nowise perplexed. Therefore in all times be firm in yoga, O Arjuna.
28The fruit of meritorious deeds, attached in the Vedas to sacrifices, to austerities, and also to almsgiving, the Yogî passeth all these by having known this, and goeth to the supreme and ancient Seat.
So I guess if you're thinking about another person, you become that person. Or if you're thinking about ice cream, you become ice cream. Or something like that.
It's all very clearly and scientifically explained here: Understanding Reincarnation from Bhagavad-gita
