Someone tells you: “I’ll bet you $1 that if you give me $3, I will give you $5 in return”. Is this a good bet?
You should not accept the bet and give him $3. If the person gives you the $5, then you would have given him $1+$3=$4, and you would make a profit of $1. However, the person can simply not give you anything and you will lose $3-$1=$2.