(childish) if you read this you're stupid (customary scribbling on school desks etc.)

sow the wind, reap the whirlwind

nothing ventured, nothing gained

in for a penny, in for a pound

with friends like these who needs enemies

he who digs a pit for others falls in himself

"He who wants to have everything, gets nothing in the end"; One cannot like everything and give equally to everyone, therefore must take sides and choose.

it is important not to waste time and opportunity; some situations require bold decisions

people who live in glass houses shouldn't throw stones

if you lie down with dogs, you get up with fleas