At 21 weeks, a baby's eyes are still closed, but he'll still be aware of light and dark. Because of this, twins can probably make out their sibling changing position and they're increasingly becoming aware of each other. It's thought that because memory starts developing around now, twin babies may begin to bond at this stage.
As ultrasound scanning has proven, there's plenty of contact between twin babies in the uterus, especially as the amount of space decreases. They have been seen to touch often, kick, and try to grab. Each twin reacts to the other's movements.
They may not act in the same way, with one twin favoring different movements than the other one. For instance, one baby may prefer sucking his thumb, while the other likes clutching his cord. Nor do they necessarily have the same body clock, so they may be active at different times. This proves that, even in the uterus, twin babies are already individuals.