What has 13 hearts but no other organs?

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.

I have a head like a cat and feet like a cat, but I am not a cat. What am I?

In 1990, a person is 15 years old. In 1995, that same person is 10 years old. How can this be?

With the help clues provided in the image, can you find the 5 digit key?

You are mixing a mixture of cement and Sand with five gallons of water. You have a garden hose giving you all the water you need. The problem is that you only have a four-gallon bucket and a seven-gallon bucket and neither has graduation marks. Find a method to measure five gallons.

A river should be crossed by a father, a mother and their two sons and two daughters.
There are some rules that should be followed while crossing the river. There can be only two people in the raft while crossing. The daughters cannot be with their father unless there is the presence of the mother. The sons cannot be with their mothers unless the father is present. Unless the guard is on the board, the criminals cannot be with any of the family members. Only the adults like the father, the guard, and the mother knows to use the raft.

How will they cross the river?

You knock a Table over in the library, and suddenly 20 eyes are looking at you.

Forwards I am heavy, backward I am not. What am I?

How many sides does a circle have?