Last week, I bought an alarm system for about $ 450. It consisted of a control unit with three small units.  I put the control unit in the sitting-room and fastened the other units by the front door, back door and living-room windows. The instructions told us to choose three numbers, so we chose 491, the last three numbers of our telephone number. Now I will explain how the alarm works: 1. There is a power siren in the control unit. It makes a very loud noise. 2. Each of the small units sends out beams or rays in different directions. If anything moves, it breaks a beam. This sends signal to the control unit. The siren makes a noise which you can hear 50 meters away. 3. When we go to bed, I press the three buttons numbered 4, 9 and 1. Then we have 30 seconds to get out of the room before the alarm starts to work. That night I slept soundly because I was sure that no burglar could get into our house. However, at about 2:10 a. m. , I woke up and heard the siren. "There must be a burglar in the house?" Mary said, "What shall we do?" "I'll go and see who's there," I said, "Stay here. Don't make a noise. " I went down stairs quietly. When I reached the living-room, I switched on my torch and looked round the room. Then I turned the light on. I switched the siren off and searched the rooms downstairs. There was nothing wrong except that the back door was unlocked. I locked it, re-set the alarm and went back to bed. About an hour later, the alarm started again. I jumped out of bed, fell over a chair in the dark and bumped into the bedroom door. Mary woke up and started hitting me with a torch. "Hey! Wait a minute!" I whispered, "It's only me. I'm going downstairs to see what's wrong." I went down into the living-room and listened for a moment. The only sound I could hear was the siren. I tamed on the light and switched the siren off. As I did so, I glanced across at the curtains in front of the windows. I saw a house lizard (a kind of small animals)  disappear behind the curtains. "Oh!" I said to myself. "That's our burglar." When the lizard moved, it started the siren. I guessed the alarm had been made in Europe, where there are no house lizards. Well, I won't finish this story but if you want an alarm system free of charge, let me know. I'll send you ours. We bought a dog this morning. It knows the difference between a lizard and a burglar . The purpose of the three units was to  _  .
sound their own siren when the control unit told them to send a message to the control unit and start the siren check that the control unit worked properly all the time give them enough light