Dawn of the Dead

After a long shift, nurse Ana returns to her suburban neighborhood and her husband, Luis. Caught up in a date night, they miss an emergency news bulletin. The next morning, the neighbour's daughter, Vivian, enters their bedroom and bites Luis, who subsequently dies and reanimates as a zombie before attacking Ana. She flees in her car but crashes and passes out. Upon waking, Ana joins with Police Sergeant Kenneth Hall, salesman Michael, petty criminal Andre and his pregnant wife Luda. They break into a nearby mall and are attacked by a zombified security guard, who infects Luda after scratching her on the arm. They are confronted by three mall guards â C.J., Bart, and Terry. The guards make them surrender their weapons in exchange for refuge. They split into groups to secure the mall and then go to the roof. They see another survivor, Andy, stranded in his gun store across the parking lot.
The next day, a truck carrying survivors enters the lot, pursued by zombies. C.J. and Bart refuse to let the survivors in, but are overruled and disarmed. The newcomers include Norma, Steve Marcus, Tucker, Monica, Glen, Frank and his daughter, Nicole. A sick woman is wheeled inside by wheelbarrow, only to quickly die and reanimate before getting killed by Ana. The group determines that the infection is passed by bites. Andre leaves to see Luda, who has kept her condition hidden from the group. They realize that Frank has been bitten and is a potential threat. Frank elects to be isolated. When he dies and turns, Kenneth shoots him.
Over time, Kenneth and Andy start communicating by writing messages on a whiteboard; relationships escalate between Ana and Michael as does Nicole and Terry. One day, the power goes out. CJ, Bart, Michael, and Kenneth enter the parking garage to activate the emergency generator and find a friendly dog inside. Moments later, Bart is attacked by a zombie that attracts more of them; which leads to him getting consequently killed and forcing the others to retreat after setting the zombies on fire. Meanwhile, Luda, who has already been tied up by Andre, dies and reanimates before giving birth the moment Norma enters to check on the couple. Norma shoots Luda dead, promoting Andre to snap and retaliate; they exchange gunfire and both are killed. The others arrive to find a zombie baby, which Ana executes immediately.
Concluding that staying in the mall is not a long-term choice, the group decides to escape. They plan to get to the local marina and travel on Steve's yacht to an island on Lake Michigan. They reinforce two shuttle buses in the parking garage and pack weapons and provisions on them. They also plan to pick up Andy. They strap food and a walkie-talkie onto the dog, Chips, and lower him into the parking lot. Chips gets into Andy's store safely, but a zombie follows through the dog door and bites Andy. Nicole, worrying about Chips, crashes the truck into the gun store and is trapped by a now-zombified Andy. Kenneth, Michael, Tucker, Terry, and C.J. reach the gun store via the sewers and rescue Nicole and Chips after killing Andy. They then grab ammunition and go back to the mall. Along the way, Tucker falls and breaks his legs; allowing the oncoming zombies to attack him before C.J. mercifully kills him. Once inside, they are unable to lock the door, forcing a premature evacuation.
Everyone boards the buses. When fighting the horde, Glen loses control of a chainsaw, accidentally killing himself and Monica. Blood splatters on the windshield, causing the bus to crash. Surviving the crash, Steve abandons the group and attempts to escape alone, but is quickly attacked and killed by a zombie. He shortly returns as a zombie and is killed by Ana, who retrieves his boat keys before narrowly avoiding the persuing zombies to regroup with the others. After taking the remaining bus to the marina, C.J. is cornered by the zombies; forcing him to sacrifice himself so that the others can escape. Michael subsequently reveals a bite wound on his arm and commits suicide just as Ana, Kenneth, Nicole, Terry, and Chips flee on the yacht.
The movie ends with footage from Steve's camcorder. It shows the group running out of supplies before finally arriving at an island. There, they are exposed and persued by a swarm of zombies. The camcorder is dropped, recording dozens of zombies chasing them, leaving their fate unknown.

Question: How does Kenneth and Andy communicate?
Answer: by writing messages on a whiteboard