1. stratus
   cumulus
2. cirrus

Cirrus clouds are often wispy, thin clouds that have a strand-like appearance. They are often so long and transparent that one cloud seems to stretch on for many miles.

1. electrical system
   ventilation system
2. suspension system

Shock absorbers, struts and the like are all part of a vehicle's suspension system. They're the parts that keep potholes from rattling the fillings out of your teeth.

1. AB
   A
2. T

There are four human blood types, O, AB, A and B. O-negative is the universal blood donor type because it is compatible with other blood types.

1. water pump
   speedometer
2. protractor

A protractor is one of those nifty transparent plastic tools that's used for measuring angles. It's loaded with lines and numbers that indicate degrees.

1. 5
   10
2. 1,000

The kilogram is a standard unit of measure, equal to about 2.2 pounds. There are 1,000 grams in each kilogram.

1. an infant
   a liar
2. a priest

A person who is duplicitous engages in deceptive behavior. Liars are duplicitous people.

1. hemisphere
   troposphere
2. stratosphere

Just above the troposphere, you'll find the stratosphere … and that's also the region that contains the ozone layer. Ozone helps to absort ultraviolet light from the sun, and thus keeps us from getting fried by the sun's immense energy.

1. Earth
   Neptune
2. Mercury

Sun-blasted on one side, frozen on the other, Mercury is the closest planet to the sun. Earth is the third-closest planet, right after Venus.

1. cat lasers
   inertia
2. the Supreme Court

Newton’s First Law of Motion says that objects in motion tend to stay in motion. In other words, it's the law of inertia.

1. a type of art
   the universe's creation
2. how babies are made

It's also what happens when you play with matches and gasoline. Many scientists posit that the universe began with a massive explosion -- the Big Bang, about 14 billion years ago.

1. nervous
   transmission
2. circulatory

In the human body, the vascular system is the circulatory system. It's the web of vessels that move blood and lymph around the body.

1. capacitor
   resistor
2. transformer

Capacitors are electrical components that can store an electrical charge. In the early days of electricity, capacitors were called condensers.

1. meat
   plants
2. people

Animals like cows are herbivores. They eat plants and only plants, except that one time when Betsy came down with Mad Cow disease and wound up eating Farmer John's face, but that hardly ever happens.

1. parasitism
   mutualism
2. commercialization

It happens in ecosystems around the world -- parasitism. Tapeworms, for example, invade the digestive systems of a host creature and use its energy to sustain their existence.

1. lead
   copper
2. steel

Copper is a very conductive metal. It's why most wires use copper as their base metal.

1. creating electrical circuits
   flying into enemy territory
2. removing thin strips of wood

Planes are specialized tools in a woodworking shop. They help carpenters remove thin strips of wood from a piece of lumber.

1. You grow it.
   You burn it.
2. You smash it.

Incinerate means to destroy an object by burning it. Better to incinerate those biohazardous materials than to dump them into the local forest.

1. arteries
   veins
2. capillaries

Blood carrying fresh oxygen flows out of the heart via arteries. Veins, on the other hand, direct unoxygenated blood back to the heart.

1. succulent
   evergreen
2. deciduous

In the Northern Hemisphere, each autumn, deciduous trees shed their leaves as part of their strategy for surviving cold winters. Then, in the spring, they regrow their leaves.

1. triple circle
   square
2. double helix

The intertwining double helix structure of DNA is iconic. Its delicate strands carry the genetic code that makes us who we are.

1. braking system
   steering system
2. transmission system

The clutch and drive shaft are part a car's transmission system. Without a transmission, that car isn't going anywhere.

1. separate
   tiny
2. private

Something that's discrete is separate or individual. One beach is made of billions of discrete bits of sand.

1. the object with greater mass
   the object with less mass
2. They'll strike the ground at the same time.

In the absence of air resistance, the objects will strike the ground at the same time. That's because gravity exerts the same force on both objects.

1. sun zit
   solar flare
2. Pontiac Sunfire

A solar flare is an abrupt eruption of high-energy radiation from one part of the sun's surface. Solar flares often cause interference in electrical devices and communications on Earth.

1. lovable
   rotting
2. on fire

Putrid refers to something that's rotting and emitting a terrible odor. Your high school son's sneakers are putrid.

1. sun's orbit of Earth
   tilt of Earth's rotational axis
2. Mother Nature

Earth's rotational axis has a tilt that causes seasons to change. It's why the Northern Hemisphere burns up in July and then plunges into a deep freeze in January.

1. consume
   obliterate
2. condone

If you condone something, you tolerate it. "She condoned her toddler's terrible behavior at the mall, something other shoppers could barely tolerate."

1. sodium chloride
   Vitamin C
2. chia

Sodium chloride is a mineral. Better known as salt, it's a substance that the human body needs in order to function properly.

1. 0
   7
2. 19

In chemistry, the pH scale is used to note whether a solution is basic or acid. At 7, the solution is neutral -- below 7, it is considered acidic.

1. complex
   contrast
2. stupid

Something that's imbecilic has the traits of an imbecile -- that is, it's stupid, silly, or both. Try not to be imbecilic.