That you see the flash for an instant, while the thunder can be audible for a few seconds has two reasons. Firstly, the speed of light (300.000 km/s) is much larger than the speed of sound (330 m/s). Therefore, light of the flash will arive first, and then the sound. The second reason is that the average flash does not take a straight way. Different points on the flash lie at different distances away from you. If for example, the bottom part of the flash is nearer to you than the top part, the light from the bottom part arrives first in your eyes, and thereafter the light of the top part. Because the speed of light is so high, you do not recognize the difference in arrival times of light an you see the flash instanteously. But the speed of sound is much lower. If for example the distance between you and the top part is about 1000 m larger than the distance between you and the bottom part, you will hear about 3 seconds of thunder because the light from the top part arrives 3 seconds later in your ears than that of the bottom part.