Do fish sleep during the day or at night？

Sleep is the rest of the brain. Since fish have no brain, where they rest is called sleep.

When it thunders, why do you see the lightning first, then hear the thunder?

300000km/s > 300m/s

Does a perm keep your hair curly forever

Normal hair transplants are unlikely to become permanently crooked, because curly hair is hereditary

Why do helicopters have to have small propellers at the back？

The tail rotor is designed to counteract the torque generated by the main propeller, which would otherwise spin as soon as the helicopter leaves the ground. The tail rotor is separated by the main shaft of the propeller, and its speed is proportional to the main propeller. The ratio stays the same.

The concept of gravity was discovered by which famous physicist？

How many colors are there in the rainbow？