Isaac's tears decelerate as they travel. Upon stopping or hitting an obstacle, they burst into 8 tears in all directions.
Hitting an enemy will not cause them to burst.
Tears can be fired into other tears to feed them, increasing their damage and the damage of the burst tears.
If one tear is fed 5 other tears, it will immediately burst.
The tears will have their damage added together, with the first consumed tear giving additional damage.
Tears in the burst will deal half the damage the main tear would have.
Tear bursting can also happen from sheer shot size. With sufficiently high damage or several tear-size increasing items, the tears will burst almost immediately after firing.