Cortex Scan - This is your basic mind reading ability. It serves to refill your Concentration Meter and also comes in handy when trying to learn information from certain characters. You can do a Cortex Scan on any living creature.
Psychokinesis (PK) - This allows you to pick up objects and toss them. At first, you can only pick up people, but will eventually be able to pick up cars, tanks, robots, etc.
Concentration Meter - Anytime you use certain powers, like Brain Extraction or Psychokinesis, it uses your Concentration. Your Concentration Meter will empty as you use these powers. It will refill on its own eventually, or, you can Cortex Scan (Mind Read) any living creature to quickly fill it up.
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Brain Extraction - You can extract the brain of most humans using your Brain Extraction button. Brains give you various amounts of DNA. You can either kill a human first using your Zap-O-Matic and then extract his/her brain, or you can simply use the Brain Extraction button directly on a living human to suck the brain right out of them. This direct extraction, if you will, will not work (or is much harder to do) on humans with more intelligence, namely policeman, soldiers, and Majestic Agents. Also, I noticed in some levels that direct extraction yields higher amounts of DNA. This is also true for smart humans. For example, a Scientists brain will usually be worth more DNA than an average Joe.
Hypno Blast - You an hypnotize people at any time. To do so, target them and hold down your Hypno button. When they are hypnotized, you'll get two options, Sleep or Distraction. Doing this will allow you to put enemies to sleep for a limited period of time or allow you to cause them to make a distraction so you can slip by or grab something that is being guarded. There are also certain specific cases where you'll need to hypnotize somebody to make them return to your ship. In these instances, the option that you need will pop up for you.
DNA - This is your basic form of currency in the game. You use DNA to purchase upgrades to your weapons, abilities, and Saucer. There will also be times when it's necessary to gather "X" amount of DNA before a new mission will become unlocked.
Health Meter - The health meter is in the upper right of the screen. When you are on the ground, as Crypto, your health will regenerate. If you are in the Saucer, it will not and you'll have to find Repair Pods to heal your Saucer. Also, in some levels, you'll see green emerald pickups. This is health. Yes, Crypto does regenerate his own health, but sometimes these green pickups come in very handy.
Enemy Health Meter - Anytime that you target an individual, building, car, etc., you'll see a Red Health Meter pop in below your green health meter. This is the health of the object/person you are currently targeting. It is helpful to see how much life a person or object has remaining so you can figure out how much longer it'll take to kill/destroy.
Weapons - The weapon you currently have selected is in the bottom right of the screen. I describe each weapon as you earn it, in the walkthrough. In general, you have your basic weapon that doesn't use ammo, but instead, regenerates. Other weapons that you gain often require ammo, of which there is usually plenty scattered throughout any one level.
Radar - Your radar is in the bottom right of the screen. It has lots of helpful information on it. Pink diamonds are your current goals, blue dots are pedestrians, "!" are people who are tipped off to your presence, red dots are enemies, and the blue home icon represents landing sites (if in the Saucer) or where your Saucer is if you are on the ground.
Holobob - Holobobbing (the verb) is when you take on the image of another person to disguise yourself and blend in. Place your reticle (target) over any person and push the Holobob button and you will take their appearance. Doing this will drain your Concentration Meter. To keep up the disguise, simply Cortex Scan people as you go. While disguised, you cannot pick up objects or fire your weapons. You can, however, use any of your mind powers, such as PK to kill enemies. Note that Majestic can see through your disguise. While holobobbed, if you see yourself flash red, it means there is a Majestic Agent nearby. Run away from him as quickly as possible or you disguise will be blown.
Pink Beams of Light - A thin column of pink light represents where you need to go for the current mission.
Yellow Beams of Light - A thin column of yellow light represents a Side Mission. Walk into the light to start the side mission.
Jet Pack - Fairly self-explanatory. Note, the green/red lights on the back of the Jet Pack indicate how much fuel you have left.
Side Missions - Side missions are marked with a Yellow Beam of Light. They are all fairly easy to accomplish and they earn you DNA. They are a good way to build up your DNA quickly. There is no reason for me to list them as they are self explanatory and, like I said, easy. They also serve as land marks, so get familiar with them, especially when you're Probe hunting.