“save for a rainy day” = save something for the future when it is needed
“Instead of spending money on a new TV, I’m going to save it for a rainy day. You never know when you’ll need it.”

“a dime a dozen” = very common, usually inexpensive, and easy to find
“I bought my dad a new watch at a flea market, and I thought I got a great deal only to discover they were a dime a dozen at other places.”

“make your mouth water” = make you hungry by just looking at or smelling food
“I’m so hungry that just looking at the fruit makes my mouth water.”

“easy as pie” = not difficult or requiring much effort
“Making delicious cookies is easy as pie.”

whip up (phrasal verb): prepare or cook food quickly
– We don’t have much time for dinner, so I’ll whip up something right away.

“keep in touch” = stay in contact or communications with someone
“Using popular social media Web sites is one way to keep in touch with family and friends.”

“reach someone” = be able to contact or talk with someone
“If you want to talk, you can reach me by phone, by email, through FaceBook.”

“do the dishes” = wash dishes
“If you want to earn allowance this week, you need to do the dishes everyday.”

“in the bucket” = a very small amount
“I’ve been saving my allowance for two months, but it’s still only a drop in the bucket, and it’ll take me forever to save enough money for a new bike.”

“catch a train” = get on a train
“I’m planning on catching the train at 3:10, so I don’t want to be late.”

“hold up traffic” = slow traffic
“The train tends to hold up traffic when it crosses this busy road in the morning.”

not make heads or tails of (idiom): not be able to understand something at all 
– I can’t make heads or tails of this train map.

“call someone up” = telephone someone
“Hey, call me up if you want to get together.”

“call on someone” = drop by and visit
“I’ll call on you tomorrow around noon.”