Sony SGPT21 - Manual (Page 137)

Heat builds up in the tablet device, the battery, and the AC adapter
while the tablet device is in use or being charged.
Heat builds up in the tablet device, the battery, and the AC adapter while the tablet device is in use or being
charged. This is normal and should not affect the performance of your tablet device.
The tablet device can be shut down for safety when overheated. In this case, use the tablet device in a cool
area after the tablet device cools down.
If your tablet device becomes abnormally hot, it may be in need of repair. Turn off your tablet device, unplug
the AC power cord (mains lead) from the AC adapter, and stop using the tablet device. Contact the
service/support center named in the warranty.