put on (pounds)

I've noticed that I've been putting on a few pounds.

To 'put on' pounds is to increase your weight.  Notice the following:

1. She put on pounds after her first child.
   
2. It's easy to put on pounds when you don't exercise.
   

lose weight

Playing sports helps to lose the weight that you might have gained.

To 'lose weight' is to decrease your weight.  Notice the following:

1. She lost a lot of weight when she started this job.
   
2. Are you trying to lose weight?
   

(not to) snack

I eat three meals a day and I try not to snack in between.

To 'snack' is to eat food between the normal meals of breakfast, lunch and dinner.  Notice the following:

1. We should buy some food to snack on.
   
2. Some people think it's better to snack than eat big meals.
   

junk food

Actually, my problem is that I like junk food.

'Junk food' is food that is not nutritious or very bad for us to eat.  It is usally high in fat, sugar or salt.  Notice the following:

1. She almost never eats junk food.
   
2. What kind of junk food do you like?
   

tough

I think having to eat less is going to be tough.

Something that is 'tough' to do is difficult. Notice the following:

1. The final exam is really tough.
   
2. It's tough to go to school and have a job.